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+<meta http-equiv="content-type" content="text/html; charset=utf-8">
+<meta http-equiv="Content-Style-Type" content="text/css">
+<title>The Project Gutenberg eBook of On a Dynamical Top, by James Clerk Maxwell</title>
+
+<style type="text/css">
+
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+h1, h2, h3, h4, h5 {text-align: center; font-style: normal; font-weight:
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+
+<div style='text-align:center; font-size:1.2em; font-weight:bold;'>The Project Gutenberg eBook of On a Dynamical Top, by James Clerk Maxwell</div>
+<div style='display:block;margin:1em 0'>
+This eBook is for the use of anyone anywhere in the United States and
+most other parts of the world at no cost and with almost no restrictions
+whatsoever. You may copy it, give it away or re-use it under the terms
+of the Project Gutenberg License included with this eBook or online
+at <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you
+are not located in the United States, you will have to check the laws of the
+country where you are located before using this eBook.
+</div>
+<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: On a Dynamical Top</div>
+<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Author: James Clerk Maxwell</div>
+<div style='display:block;margin:1em 0'>Release Date: June 1, 2002 [eBook #5192]<br>
+[Most recently updated: January 21, 2021]</div>
+<div style='display:block;margin:1em 0'>Language: English</div>
+<div style='display:block;margin:1em 0'>Character set encoding: UTF-8</div>
+<div style='display:block; margin-left:2em; text-indent:-2em'>Produced by: Gordon Keener</div>
+<div style='margin-top:2em;margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK ON A DYNAMICAL TOP ***</div>
+
+<h1>On a Dynamical Top,</h1>
+
+<h3>
+for exhibiting the phenomena of the motion
+of a system of invariable form about a fixed point, with some
+suggestions as to the Earth&rsquo;s motion</h3>
+<h2>James Clerk Maxwell</h2>
+<P ALIGN="CENTER"><STRONG>[From the <i>Transactions of the Royal Society of Edinburgh</i>,
+Vol. XXI. Part IV.]
+<BR>(Read 20th April, 1857.)</STRONG></P>
+
+<P>
+To those who study the progress of exact science, the common
+spinning-top is a symbol of the labours and the perplexities of men
+who had successfully threaded the mazes of the planetary motions. The
+mathematicians of the last age, searching through nature for problems
+worthy of their analysis, found in this toy of their youth, ample
+occupation for their highest mathematical powers.
+
+<P>
+No illustration of astronomical precession can be devised more perfect
+than that presented by a properly balanced top, but yet the motion of
+rotation has intricacies far exceeding those of the theory of
+precession.
+
+<P>
+Accordingly, we find Euler and D&rsquo;Alembert devoting their talent and
+their patience to the establishment of the laws of the rotation of
+solid bodies. Lagrange has incorporated his own analysis of the
+problem with his general treatment of mechanics, and since his time
+M. Poinsôt has brought the subject under the power of a more
+searching analysis than that of the calculus, in which ideas take the
+place of symbols, and intelligible propositions supersede equations.
+
+<P>
+In the practical department of the subject, we must notice the
+rotatory machine of Bohnenberger, and the nautical top of Troughton.
+In the first of these instruments we have the model of the Gyroscope,
+by which Foucault has been able to render visible the effects of the
+earth&rsquo;s rotation. The beautiful experiments by which Mr J. Elliot has
+made the ideas of precession so familiar to us are performed with a
+top, similar in some respects to Troughton&rsquo;s, though not borrowed from
+his.
+
+<P>
+The top which I have the honour to spin before the Society, differs
+from that of Mr Elliot in having more adjustments, and in being
+designed to exhibit far more complicated phenomena.
+
+<P>
+The arrangement of these adjustments, so as to produce the desired
+effects, depends on the mathematical theory of rotation. The method
+of exhibiting the motion of the axis of rotation, by means of a
+coloured disc, is essential to the success of these adjustments. This
+optical contrivance for rendering visible the nature of the rapid
+motion of the top, and the practical methods of applying the theory
+of rotation to such an instrument as the one before us, are the
+grounds on which I bring my instrument and experiments before the
+Society as my own.
+
+<P>
+I propose, therefore, in the first place, to give a brief outline of
+such parts of the theory of rotation as are necessary for the
+explanation of the phenomena of the top.
+
+<P>
+I shall then describe the instrument with its adjustments, and the
+effect of each, the mode of observing of the coloured disc when the
+top is in motion, and the use of the top in illustrating the
+mathematical theory, with the method of making the different
+experiments.
+
+<P>
+Lastly, I shall attempt to explain the nature of a possible variation
+in the earth&rsquo;s axis due to its figure. This variation, if it exists,
+must cause a periodic inequality in the latitude of every place on the
+earth&rsquo;s surface, going through its period in about eleven months. The
+amount of variation must be very small, but its character gives it
+importance, and the necessary observations are already made, and only
+require reduction.
+
+<H2>On the Theory of Rotation.</H2>
+
+<P>
+The theory of the rotation of a rigid system is strictly deduced from
+the elementary laws of motion, but the complexity of the motion of the
+particles of a body freely rotating renders the subject so intricate,
+that it has never been thoroughly understood by any but the most
+expert mathematicians. Many who have mastered the lunar theory have
+come to erroneous conclusions on this subject; and even Newton has
+chosen to deduce the disturbance of the earth&rsquo;s axis from his theory
+of the motion of the nodes of a free orbit, rather than attack the
+problem of the rotation of a solid body.
+
+<P>
+The method by which M. Poinsôt has rendered the theory more
+manageable, is by the liberal introduction of &ldquo;appropriate ideas,&rdquo;
+chiefly of a geometrical character, most of which had been rendered
+familiar to mathematicians by the writings of Monge, but which then
+first became illustrations of this branch of dynamics. If any further
+progress is to be made in simplifying and arranging the theory, it
+must be by the method which Poinsôt has repeatedly pointed out as
+the only one which can lead to a true knowledge of the subject,--that
+of proceeding from one distinct idea to another instead of trusting to
+symbols and equations.
+
+<P>
+An important contribution to our stock of appropriate ideas and
+methods has lately been made by Mr R. B. Hayward, in a paper, &ldquo;On a
+Direct Method of estimating Velocities, Accelerations, and all similar
+quantities, with respect to axes, moveable in any manner in Space.&rdquo;
+(<i>Trans. Cambridge Phil. Soc</i> Vol. x. Part I.)
+
+<P>
+<a href="#linknote-1" name="linknoteref-1" id="linknoteref-1"><big>*</big></a>
+In this communication I intend to
+confine myself to that part of the subject which the top is intended
+io illustrate, namely, the alteration of the position of the axis in a
+body rotating freely about its centre of gravity. I shall, therefore,
+deduce the theory as briefly as possible, from two considerations
+only,--the permanence of the original <i>angular momentum</i> in
+direction and magnitude, and the permanence of the original <i>vis
+viva</i>.
+
+<P>
+<a href="#linknote-2" name="linknoteref-2" id="linknoteref-2"><big>*</big></a>
+The mathematical difficulties of the theory of
+rotation arise chiefly from the want of geometrical illustrations and
+sensible images, by which we might fix the results of analysis in our
+minds.
+
+<P>
+It is easy to understand the motion of a body revolving about a fixed
+axle. Every point in the body describes a circle about the axis, and
+returns to its original position after each complete revolution. But
+if the axle itself be in motion, the paths of the different points of
+the body will no longer be circular or re-entrant. Even the velocity
+of rotation about the axis requires a careful definition, and the
+proposition that, in all motion about a fixed point, there is always
+one line of particles forming an instantaneous axis, is usually given
+in the form of a very repulsive mass of calculation. Most of these
+difficulties may be got rid of by devoting a little attention to the
+mechanics and geometry of the problem before entering on the
+discussion of the equations.
+
+<P>
+Mr Hayward, in his paper already referred to, has made great use of
+the mechanical conception of Angular Momentum.
+
+<P>
+<P>
+<DIV><B>Definition 1</B>
+The Angular Momentum of a particle about an axis is measured by the
+product of the mass of the particle, its velocity resolved in the
+normal plane, and the perpendicular from the axis on the direction of
+motion.</DIV><P></P>
+
+<P>
+<a href="#linknote-3" name="linknoteref-3" id="linknoteref-3"><big>*</big></a>
+The angular momentum of any system about an axis is
+the algebraical sum of the angular momenta of its parts.
+
+<P>
+As the <i>rate of change</i> of the <i>linear momentum</i> of a
+particle measures the <i>moving force</i> which acts on it, so the
+<i>rate of change</i> of <i>angular momentum</i> measures the
+<i>moment</i> of that force about an axis.
+
+<P>
+All actions between the parts of a system, being pairs of equal and
+opposite forces, produce equal and opposite changes in the angular
+momentum of those parts. Hence the whole angular momentum of the
+system is not affected by these actions and re-actions.
+
+<P>
+<a href="#linknote-4" name="linknoteref-4" id="linknoteref-4"><big>*</big></a>
+When a system of invariable form revolves about an
+axis, the angular velocity of every part is the same, and the angular
+momentum about the axis is the product of the <i>angular velocity</i>
+and the <i>moment of inertia</i> about that axis.
+
+<P>
+<a href="#linknote-5" name="linknoteref-5" id="linknoteref-5"><big>*</big></a>
+It is only in particular cases, however, that the
+<i>whole</i> angular momentum can be estimated in this way. In
+general, the axis of angular momentum differs from the axis of
+rotation, so that there will be a residual angular momentum about an
+axis perpendicular to that of rotation, unless that axis has one of
+three positions, called the principal axes of the body.
+
+<P>
+By referring everything to these three axes, the theory is greatly
+simplified. The moment of inertia about one of these axes is greater
+than that about any other axis through the same point, and that about
+one of the others is a minimum. These two are at right angles, and
+the third axis is perpendicular to their plane, and is called the mean
+axis.
+
+<P>
+<a href="#linknote-6" name="linknoteref-6" id="linknoteref-6"><big>*</big></a>
+Let <IMG
+ WIDTH="21" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img1.png"
+ ALT="$A$">, <IMG
+ WIDTH="22" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img2.png"
+ ALT="$B$">, <IMG
+ WIDTH="22" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img3.png"
+ ALT="$C$"> be the moments of inertia about the
+principal axes through the centre of gravity, taken in order of
+magnitude, and let <IMG
+ WIDTH="26" HEIGHT="36" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img4.png"
+ ALT="$\omega_1$"> <IMG
+ WIDTH="26" HEIGHT="36" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img11.png"
+ ALT="$\omega_2$"> <IMG
+ WIDTH="26" HEIGHT="36" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img12.png"
+ ALT="$\omega_3$"> be the angular
+velocities about them, then the angular momenta will be <IMG
+ WIDTH="40" HEIGHT="37" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img13.png"
+ ALT="$A\omega_1$">,
+<IMG
+ WIDTH="41" HEIGHT="37" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img14.png"
+ ALT="$B\omega_2$">, and <IMG
+ WIDTH="41" HEIGHT="37" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img15.png"
+ ALT="$C\omega_3$">.
+
+<P>
+Angular momenta may be compounded like forces or velocities, by the
+law of the &ldquo;parallelogram,&rdquo; and since these three are at right angles
+to each other, their resultant is
+<BR>
+<DIV ALIGN="RIGHT">
+
+<!-- MATH
+ \begin{equation}
+\sqrt{A^2\omega_1^2 + B^2\omega_2^2 + C^2\omega_3^2} = H
+\end{equation}
+ -->
+<TABLE WIDTH="100%" ALIGN="CENTER">
+<TR VALIGN="MIDDLE"><TD ALIGN="CENTER" NOWRAP><IMG
+ WIDTH="397" HEIGHT="36" BORDER="0"
+ SRC="images/img16.png"
+ ALT="\begin{displaymath}
+\sqrt{A^2\omega_1^2 + B^2\omega_2^2 + C^2\omega_3^2} = H
+\end{displaymath}"></TD>
+<TD WIDTH=10 ALIGN="RIGHT">
+(1)</TD></TR>
+</TABLE>
+<BR CLEAR="ALL"></DIV><P></P>
+and this must be constant, both in magnitude and direction in space,
+since no external forces act on the body.
+
+<P>
+We shall call this axis of angular momentum the <i>invariable
+axis</i>. It is perpendicular to what has been called the invariable
+plane. Poinsôt calls it the axis of the couple of impulsion. The
+<i>direction-cosines</i> of this axis in the body are,
+
+<P>
+<BR><P></P>
+<DIV ALIGN="CENTER">
+<!-- MATH
+ \begin{displaymath}
+\begin{array}{c c c}
+\displaystyle l = \frac{A\omega_1}{H}, &
+\displaystyle m = \frac{B\omega_2}{H}, &
+\displaystyle n = \frac{C\omega_3}{H}.
+\end{array}
+\end{displaymath}
+ -->
+
+<IMG
+ WIDTH="415" HEIGHT="46" BORDER="0"
+ SRC="images/img17.png"
+ ALT="\begin{displaymath}
+\begin{array}{c c c}
+\displaystyle l = \frac{A\omega_1}{H}, ...
+...ga_2}{H}, &amp;
+\displaystyle n = \frac{C\omega_3}{H}.
+\end{array}\end{displaymath}">
+</DIV>
+<BR CLEAR="ALL">
+<P></P>
+
+<P>
+Since <IMG
+ WIDTH="16" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img18.png"
+ ALT="$I$">, <IMG
+ WIDTH="23" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img19.png"
+ ALT="$m$"> and <IMG
+ WIDTH="18" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img20.png"
+ ALT="$n$"> vary during the motion, we need some additional
+condition to determine the relation between them. We find this in the
+property of the <i>vis viva</i> of a system of invariable form in
+which there is no friction. The <i>vis viva</i> of such a system must
+be constant. We express this in the equation
+
+<P>
+<BR>
+<DIV ALIGN="RIGHT">
+
+<!-- MATH
+ \begin{equation}
+A\omega_1^2 + B\omega_2^2 + C\omega_3^2 = V
+\end{equation}
+ -->
+<TABLE WIDTH="100%" ALIGN="CENTER">
+<TR VALIGN="MIDDLE"><TD ALIGN="CENTER" NOWRAP><IMG
+ WIDTH="377" HEIGHT="33" BORDER="0"
+ SRC="images/img21.png"
+ ALT="\begin{displaymath}
+A\omega_1^2 + B\omega_2^2 + C\omega_3^2 = V
+\end{displaymath}"></TD>
+<TD WIDTH=10 ALIGN="RIGHT">
+(2)</TD></TR>
+</TABLE>
+<BR CLEAR="ALL"></DIV><P></P>
+
+<P>
+Substituting the values of <IMG
+ WIDTH="26" HEIGHT="36" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img4.png"
+ ALT="$\omega_1$">, <IMG
+ WIDTH="26" HEIGHT="36" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img11.png"
+ ALT="$\omega_2$">, <IMG
+ WIDTH="26" HEIGHT="36" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img12.png"
+ ALT="$\omega_3$"> in terms
+of <IMG
+ WIDTH="13" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img22.png"
+ ALT="$l$">, <IMG
+ WIDTH="23" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img19.png"
+ ALT="$m$">, <IMG
+ WIDTH="18" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img20.png"
+ ALT="$n$">,
+
+<P>
+<BR><P></P>
+<DIV ALIGN="CENTER">
+<!-- MATH
+ \begin{displaymath}
+\frac{l^2}{A} + \frac{m^2}{B} + \frac{n^2}{C} = \frac{V}{H^2}.
+\end{displaymath}
+ -->
+
+<IMG
+ WIDTH="367" HEIGHT="48" BORDER="0"
+ SRC="images/img23.png"
+ ALT="\begin{displaymath}
+\frac{l^2}{A} + \frac{m^2}{B} + \frac{n^2}{C} = \frac{V}{H^2}.
+\end{displaymath}">
+</DIV>
+<BR CLEAR="ALL">
+<P></P>
+
+<P>
+Let <IMG
+ WIDTH="81" HEIGHT="41" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img24.png"
+ ALT="$1/A = a^2$">, <IMG
+ WIDTH="81" HEIGHT="41" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img25.png"
+ ALT="$1/B = b^2$">, <IMG
+ WIDTH="74" HEIGHT="41" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img26.png"
+ ALT="$1/c = c^2$">, <IMG
+ WIDTH="95" HEIGHT="41" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img27.png"
+ ALT="$V/H^2 = e^2$">, and this
+equation becomes
+<BR>
+<DIV ALIGN="RIGHT">
+
+<!-- MATH
+ \begin{equation}
+a^2l^2 + b^2m^2 + c^2n^2 = e^2
+\end{equation}
+ -->
+<TABLE WIDTH="100%" ALIGN="CENTER">
+<TR VALIGN="MIDDLE"><TD ALIGN="CENTER" NOWRAP><IMG
+ WIDTH="376" HEIGHT="30" BORDER="0"
+ SRC="images/img28.png"
+ ALT="\begin{displaymath}
+a^2l^2 + b^2m^2 + c^2n^2 = e^2
+\end{displaymath}"></TD>
+<TD WIDTH=10 ALIGN="RIGHT">
+(3)</TD></TR>
+</TABLE>
+<BR CLEAR="ALL"></DIV><P></P>
+and the equation to the cone, described by the invariable axis within
+the body, is
+
+<P>
+<BR>
+<DIV ALIGN="RIGHT">
+
+<!-- MATH
+ \begin{equation}
+(a^2 - e^2) x^2 + (b^2 - e^2) y^2 + (c^2 - e^2) z^2 = 0
+\end{equation}
+ -->
+<TABLE WIDTH="100%" ALIGN="CENTER">
+<TR VALIGN="MIDDLE"><TD ALIGN="CENTER" NOWRAP><IMG
+ WIDTH="453" HEIGHT="33" BORDER="0"
+ SRC="images/img29.png"
+ ALT="\begin{displaymath}
+(a^2 - e^2) x^2 + (b^2 - e^2) y^2 + (c^2 - e^2) z^2 = 0
+\end{displaymath}"></TD>
+<TD WIDTH=10 ALIGN="RIGHT">
+(4)</TD></TR>
+</TABLE>
+<BR CLEAR="ALL"></DIV><P></P>
+
+<P>
+The intersections of this cone with planes perpendicular to the
+principal axes are found by putting <IMG
+ WIDTH="17" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img30.png"
+ ALT="$x$">, <IMG
+ WIDTH="17" HEIGHT="36" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img31.png"
+ ALT="$y$">, or <IMG
+ WIDTH="16" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img32.png"
+ ALT="$z$">, constant in this
+equation. By giving <IMG
+ WIDTH="15" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img33.png"
+ ALT="$e$"> various values, all the different paths of
+the pole of the invariable axis, corresponding to different initial
+circumstances, may be traced.
+
+<P>
+
+<P></P>
+<DIV ALIGN="CENTER"><A NAME="63"></A>
+<TABLE>
+<CAPTION ALIGN="BOTTOM"><STRONG>Figure:</STRONG>
+</CAPTION>
+<TR><TD><!-- MATH
+ $\includegraphics[width=\textwidth]{fig1.png}$
+ --><img SRC="images/fig1.png" alt="Figure 1">
+</TD></TR>
+</TABLE>
+</DIV><P></P>
+
+<P>
+<a href="#linknote-7" name="linknoteref-7" id="linknoteref-7"><big>*</big></a>
+In the figures, I have supposed <IMG
+ WIDTH="77" HEIGHT="23" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img36.png"
+ ALT="$a^2 = 100$">, <IMG
+ WIDTH="75" HEIGHT="23" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img37.png"
+ ALT="$b^2=
+107$">, and <IMG
+ WIDTH="76" HEIGHT="23" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img38.png"
+ ALT="$c^2= 110$">. The first figure represents a section of the
+various cones by a plane perpendicular to the axis of <IMG
+ WIDTH="17" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img30.png"
+ ALT="$x$">, which is
+that of greatest moment of inertia. These sections are ellipses
+having their major axis parallel to the axis of <IMG
+ WIDTH="15" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img39.png"
+ ALT="$b$">. The value of
+<IMG
+ WIDTH="23" HEIGHT="23" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img40.png"
+ ALT="$e^2$"> corresponding to each of these curves is indicated by figures
+beside the curve. The ellipticity increases with the size of the
+ellipse, so that the section corresponding to <IMG
+ WIDTH="76" HEIGHT="23" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img41.png"
+ ALT="$e^2 = 107$"> would be two
+parallel straight lines (beyond the bounds of the figure), after which
+the sections would be hyperbolas.
+
+<P>
+
+<P></P>
+<DIV ALIGN="CENTER"><A NAME="67"></A>
+<TABLE>
+<CAPTION ALIGN="BOTTOM"><STRONG>Figure:</STRONG>
+</CAPTION>
+<TR><TD><!-- MATH
+ $\includegraphics[width=\textwidth]{fig2.png}$
+ --><img SRC="images/fig2.png" alt="Figure 2">
+</TD></TR>
+</TABLE>
+</DIV><P></P>
+
+<P>
+<a href="#linknote-8" name="linknoteref-8" id="linknoteref-8"><big>*</big></a>
+The second figure represents the sections made by a
+plane, perpendicular to the <i>mean</i> axis. They are all
+hyperbolas, except when <IMG
+ WIDTH="76" HEIGHT="23" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img41.png"
+ ALT="$e^2 = 107$">, when the section is two
+intersecting straight lines.
+
+<P>
+
+<P></P>
+<DIV ALIGN="CENTER"><A NAME="72"></A>
+<TABLE>
+<CAPTION ALIGN="BOTTOM"><STRONG>Figure:</STRONG>
+</CAPTION>
+<TR><TD><!-- MATH
+ $\includegraphics[width=\textwidth]{fig3.png}$
+ --><img SRC="images/fig3.png" alt="Figure 3">
+</TD></TR>
+</TABLE>
+</DIV><P></P>
+
+<P>
+The third figure shows the sections perpendicular to the axis of least
+moment of inertia. From <IMG
+ WIDTH="76" HEIGHT="23" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img45.png"
+ ALT="$e^2 = 110$"> to <IMG
+ WIDTH="76" HEIGHT="23" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img41.png"
+ ALT="$e^2 = 107$"> the sections are
+ellipses, <IMG
+ WIDTH="76" HEIGHT="23" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img41.png"
+ ALT="$e^2 = 107$"> gives two parallel straight lines, and beyond
+these the curves are hyperbolas.
+
+<P>
+
+<P></P>
+<DIV ALIGN="CENTER"><A NAME="76"></A>
+<TABLE>
+<CAPTION ALIGN="BOTTOM"><STRONG>Figure:</STRONG>
+</CAPTION>
+<TR><TD><!-- MATH
+ $\includegraphics[width=\textwidth]{fig4.png}$
+ --><img SRC="images/fig4.png" alt="Figure 4">
+</TD></TR>
+</TABLE>
+</DIV><P></P>
+
+<P>
+<a href="#linknote-10" name="linknoteref-10" id="linknoteref-10"><big>*</big></a>
+The fourth and fifth figures show the sections of the
+series of cones made by a cube and a sphere respectively. The use of
+these figures is to exhibit the connexion between the different curves
+described about the three principal axes by the invariable axis during
+the motion of the body.
+
+<P>
+
+<P></P>
+<DIV ALIGN="CENTER"><A NAME="80"></A>
+<TABLE>
+<CAPTION ALIGN="BOTTOM"><STRONG>Figure:</STRONG>
+</CAPTION>
+<TR><TD><!-- MATH
+ $\includegraphics[width=\textwidth]{fig5.png}$
+ --><img SRC="images/fig5.png" alt="Figure 5">
+</TD></TR>
+</TABLE>
+</DIV><P></P>
+
+<P>
+<a href="#linknote-11" name="linknoteref-11" id="linknoteref-11"><big>*</big></a>
+We have next to compare the velocity of the invariable
+axis with respect to the body, with that of the body itself round one
+of the principal axes. Since the invariable axis is fixed in space,
+its motion relative to the body must be equal and opposite to that of
+the portion of the body through which it passes. Now the angular
+velocity of a portion of the body whose direction-cosines are <IMG
+ WIDTH="13" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img22.png"
+ ALT="$l$">,
+<IMG
+ WIDTH="23" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img19.png"
+ ALT="$m$">, <IMG
+ WIDTH="18" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img20.png"
+ ALT="$n$">, about the axis of <IMG
+ WIDTH="17" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img30.png"
+ ALT="$x$"> is
+
+<P>
+<BR><P></P>
+<DIV ALIGN="CENTER">
+<!-- MATH
+ \begin{displaymath}
+\frac{\omega_1}{1 - l^2} -
+ \frac{l}{1 - l^2}(l\omega_1 + m\omega_2 + n\omega-3).
+\end{displaymath}
+ -->
+
+<IMG
+ WIDTH="434" HEIGHT="48" BORDER="0"
+ SRC="images/img50.png"
+ ALT="\begin{displaymath}
+\frac{\omega_1}{1 - l^2} -
+\frac{l}{1 - l^2}(l\omega_1 + m\omega_2 + n\omega-3).
+\end{displaymath}">
+</DIV>
+<BR CLEAR="ALL">
+<P></P>
+
+<P>
+Substituting the values of <IMG
+ WIDTH="26" HEIGHT="36" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img4.png"
+ ALT="$\omega_1$">, <IMG
+ WIDTH="26" HEIGHT="36" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img11.png"
+ ALT="$\omega_2$">, <IMG
+ WIDTH="26" HEIGHT="36" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img12.png"
+ ALT="$\omega_3$">, in
+terms of <IMG
+ WIDTH="13" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img22.png"
+ ALT="$l$">, <IMG
+ WIDTH="23" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img19.png"
+ ALT="$m$">, <IMG
+ WIDTH="18" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img20.png"
+ ALT="$n$">, and taking account of equation (3), this
+expression becomes
+
+<P>
+<BR><P></P>
+<DIV ALIGN="CENTER">
+<!-- MATH
+ \begin{displaymath}
+H\frac{(a^2 - e^2)}{1 - l^2}l.
+\end{displaymath}
+ -->
+
+<IMG
+ WIDTH="332" HEIGHT="50" BORDER="0"
+ SRC="images/img51.png"
+ ALT="\begin{displaymath}
+H\frac{(a^2 - e^2)}{1 - l^2}l.
+\end{displaymath}">
+</DIV>
+<BR CLEAR="ALL">
+<P></P>
+
+<P>
+Changing the sign and putting <!-- MATH
+ $\displaystyle l =
+\frac{\omega_1}{a^2H}$
+ -->
+<IMG
+ WIDTH="76" HEIGHT="53" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img52.png"
+ ALT="$\displaystyle l =
+\frac{\omega_1}{a^2H}$"> we have the angular velocity of the invariable
+axis about that of <IMG
+ WIDTH="17" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img30.png"
+ ALT="$x$">
+<BR><P></P>
+<DIV ALIGN="CENTER">
+<!-- MATH
+ \begin{displaymath}
+= \frac{\omega_1}{1 - l^2} \frac{e^2 - a^2}{a^2},
+\end{displaymath}
+ -->
+
+<IMG
+ WIDTH="348" HEIGHT="50" BORDER="0"
+ SRC="images/img53.png"
+ ALT="\begin{displaymath}
+= \frac{\omega_1}{1 - l^2} \frac{e^2 - a^2}{a^2},
+\end{displaymath}">
+</DIV>
+<BR CLEAR="ALL">
+<P></P>
+always positive about the axis of greatest moment, negative about that
+of least moment, and positive or negative about the mean axis
+according to the value of <IMG
+ WIDTH="23" HEIGHT="23" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img40.png"
+ ALT="$e^2$">. The direction of the motion in every
+case is represented by the arrows in the figures. The arrows on the
+outside of each figure indicate the direction of rotation of the body.
+
+<P>
+<a href="#linknote-12" name="linknoteref-12" id="linknoteref-12"><big>*</big></a>
+If we attend to the curve described by the pole of the
+invariable axis on the sphere in fig. 5, we shall see that the areas
+described by that point, if projected on the plane of <IMG
+ WIDTH="26" HEIGHT="36" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img55.png"
+ ALT="$yz$">, are swept
+out at the rate
+
+<P>
+<BR><P></P>
+<DIV ALIGN="CENTER">
+<!-- MATH
+ \begin{displaymath}
+\omega_1 \frac{e^2 - a^2}{a^2}.
+\end{displaymath}
+ -->
+
+<IMG
+ WIDTH="323" HEIGHT="48" BORDER="0"
+ SRC="images/img56.png"
+ ALT="\begin{displaymath}
+\omega_1 \frac{e^2 - a^2}{a^2}.
+\end{displaymath}">
+</DIV>
+<BR CLEAR="ALL">
+<P></P>
+
+<P>
+Now the semi-axes of the projection of the spherical ellipse described
+by the pole are
+
+<P>
+<BR><P></P>
+<DIV ALIGN="CENTER">
+<!-- MATH
+ \begin{displaymath}
+\sqrt{\frac{e^2 - a^2}{b^2 - a^2}}
+ \hspace{1cm}\textrm{and}\hspace{1cm}
+\sqrt{\frac{e^2 - a^2}{c^2 - a^2}}.
+\end{displaymath}
+ -->
+
+<IMG
+ WIDTH="420" HEIGHT="55" BORDER="0"
+ SRC="images/img57.png"
+ ALT="\begin{displaymath}
+\sqrt{\frac{e^2 - a^2}{b^2 - a^2}}
+\hspace{1cm}\textrm{and}\hspace{1cm}
+\sqrt{\frac{e^2 - a^2}{c^2 - a^2}}.
+\end{displaymath}">
+</DIV>
+<BR CLEAR="ALL">
+<P></P>
+
+<P>
+Dividing the area of this ellipse by the area described during one
+revolution of the body, we find the number of revolutions of the body
+during the description of the ellipse--
+
+<P>
+<BR><P></P>
+<DIV ALIGN="CENTER">
+<!-- MATH
+ \begin{displaymath}
+= \frac{a^2}{\sqrt{b^2 - a^2}\sqrt{c^2 - a^2}}.
+\end{displaymath}
+ -->
+
+<IMG
+ WIDTH="367" HEIGHT="52" BORDER="0"
+ SRC="images/img58.png"
+ ALT="\begin{displaymath}
+= \frac{a^2}{\sqrt{b^2 - a^2}\sqrt{c^2 - a^2}}.
+\end{displaymath}">
+</DIV>
+<BR CLEAR="ALL">
+<P></P>
+
+<P>
+The projections of the spherical ellipses upon the plane of <IMG
+ WIDTH="26" HEIGHT="36" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img55.png"
+ ALT="$yz$"> are
+all similar ellipses, and described in the same number of revolutions;
+and in each ellipse so projected, the area described in any time is
+proportional to the number of revolutions of the body about the axis
+of <IMG
+ WIDTH="17" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img30.png"
+ ALT="$x$">, so that if we measure time by revolutions of the body, the
+motion of the projection of the pole of the invariable axis is
+identical with that of a body acted on by an attractive central force
+varying directly as the distance. In the case of the hyperbolas in
+the plane of the greatest and least axis, this force must be supposed
+repulsive. The dots in the figures 1, 2, 3, are intended to indicate
+roughly the progress made by the invariable axis during each
+revolution of the body about the axis of <IMG
+ WIDTH="17" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img30.png"
+ ALT="$x$">, <IMG
+ WIDTH="17" HEIGHT="36" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img31.png"
+ ALT="$y$"> and <IMG
+ WIDTH="16" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img32.png"
+ ALT="$z$">
+respectively. It must be remembered that the rotation about these
+axes varies with their inclination to the invariable axis, so that the
+angular velocity diminishes as the inclination increases, and
+therefore the areas in the ellipses above mentioned are not described
+with uniform velocity in absolute time, but are less rapidly swept out
+at the extremities of the major axis than at those of the minor.
+
+<P>
+<a href="#linknote-13" name="linknoteref-13" id="linknoteref-13"><big>*</big></a>
+When two of the axes have equal moments of inertia, or
+<IMG
+ WIDTH="48" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img60.png"
+ ALT="$b = c$">, then the angular velocity <IMG
+ WIDTH="26" HEIGHT="36" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img4.png"
+ ALT="$\omega_1$"> is constant, and the
+path of the invariable axis is circular, the number of revolutions of
+the body during one circuit of the invariable axis, being
+
+<P>
+<BR><P></P>
+<DIV ALIGN="CENTER">
+<!-- MATH
+ \begin{displaymath}
+\frac{a^2}{b^2 - a^2}
+\end{displaymath}
+ -->
+
+<IMG
+ WIDTH="311" HEIGHT="50" BORDER="0"
+ SRC="images/img61.png"
+ ALT="\begin{displaymath}
+\frac{a^2}{b^2 - a^2}
+\end{displaymath}">
+</DIV>
+<BR CLEAR="ALL">
+<P></P>
+
+<P>
+The motion is in the same direction as that of the rotation, or in the
+opposite direction, according as the axis of <IMG
+ WIDTH="17" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img30.png"
+ ALT="$x$"> is that of greatest
+or of least moment of inertia.
+
+<P>
+<a href="#linknote-14" name="linknoteref-14" id="linknoteref-14"><big>*</big></a>
+Both in this case, and in that in which the three axes
+are unequal, the motion of the invariable axis in the body may be
+rendered very slow by diminishing the difference of the moments of
+inertia. The angular velocity of the axis of <IMG
+ WIDTH="17" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img30.png"
+ ALT="$x$"> about the invariable
+axis in space is
+<BR><P></P>
+<DIV ALIGN="CENTER">
+<!-- MATH
+ \begin{displaymath}
+\omega_1\frac{e^2 - a^2l^2}{a^2(1 - l^2)},
+\end{displaymath}
+ -->
+
+<IMG
+ WIDTH="334" HEIGHT="53" BORDER="0"
+ SRC="images/img63.png"
+ ALT="\begin{displaymath}
+\omega_1\frac{e^2 - a^2l^2}{a^2(1 - l^2)},
+\end{displaymath}">
+</DIV>
+<BR CLEAR="ALL">
+<P></P>
+which is greater or less than <IMG
+ WIDTH="26" HEIGHT="36" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img4.png"
+ ALT="$\omega_1$">, as <IMG
+ WIDTH="23" HEIGHT="23" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img40.png"
+ ALT="$e^2$"> is greater or less
+than <IMG
+ WIDTH="24" HEIGHT="23" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img64.png"
+ ALT="$a^2$">, and, when these quantities are nearly equal, is very
+nearly the same as <IMG
+ WIDTH="26" HEIGHT="36" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img4.png"
+ ALT="$\omega_1$"> itself. This quantity indicates the
+rate of revolution of the axle of the top about its mean position, and
+is very easily observed.
+
+<P>
+<a href="#linknote-15" name="linknoteref-15" id="linknoteref-15"><big>*</big></a>
+The <i>instantaneous axis</i> is not so easily
+observed. It revolves round the invariable axis in the same time with
+the axis of <IMG
+ WIDTH="17" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img30.png"
+ ALT="$x$">, at a distance which is very small in the case when
+<IMG
+ WIDTH="17" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img66.png"
+ ALT="$a$">, <IMG
+ WIDTH="15" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img39.png"
+ ALT="$b$">, <IMG
+ WIDTH="15" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img67.png"
+ ALT="$c$">, are nearly equal. From its rapid angular motion in
+space, and its near coincidence with the invariable axis, there is no
+advantage in studying its motion in the top.
+
+<P>
+<a href="#linknote-16" name="linknoteref-16" id="linknoteref-16"><big>*</big></a>
+By making the moments of inertia very unequal, and in
+definite proportion to each other, and by drawing a few strong lines
+as diameters of the disc, the combination of motions will produce an
+appearance of epicycloids, which are the result of the continued
+intersection of the successive positions of these lines, and the cusps
+of the epicycloids lie in the curve in which the instantaneous axis
+travels. Some of the figures produced in this way are very pleasing.
+
+<P>
+In order to illustrate the theory of rotation experimentally, we must
+have a body balanced on its centre of gravity, and capable of having
+its principal axes and moments of inertia altered in form and position
+within certain limits. We must be able to make the axle of the
+instrument the greatest, least, or mean principal axis, or to make it
+not a principal axis at all, and we must be able to <i>see</i> the
+position of the invariable axis of rotation at any time. There must
+be three adjustments to regulate the position of the centre of
+gravity, three for the magnitudes of the moments of inertia, and three
+for the directions of the principal axes, nine independent
+adjustments, which may be distributed as we please among the screws of
+the instrument.
+
+<P>
+
+<P></P>
+<DIV ALIGN="CENTER"><A NAME="132"></A>
+<TABLE>
+<CAPTION ALIGN="BOTTOM"><STRONG>Figure:</STRONG>
+</CAPTION>
+<TR><TD>
+<DIV ALIGN="CENTER">
+<!-- MATH
+ $\includegraphics[width=0.8\textwidth]{fig6.png}$
+ --><img SRC="images/fig6.jpg" alt="Figure 6">
+</DIV></TD></TR>
+</TABLE>
+</DIV><P></P>
+
+<P>
+The form of the body of the instrument which I have found most
+suitable is that of a bell (fig. 6). <IMG
+ WIDTH="22" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img3.png"
+ ALT="$C$"> is a hollow cone of brass,
+<IMG
+ WIDTH="21" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img70.png"
+ ALT="$R$"> is a heavy ring cast in the same piece. Six screws, with heavy
+heads, <IMG
+ WIDTH="17" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img30.png"
+ ALT="$x$">, <IMG
+ WIDTH="17" HEIGHT="36" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img31.png"
+ ALT="$y$">, <IMG
+ WIDTH="16" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img32.png"
+ ALT="$z$">, <IMG
+ WIDTH="22" HEIGHT="22" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img71.png"
+ ALT="$x'$">, <IMG
+ WIDTH="21" HEIGHT="39" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img72.png"
+ ALT="$y'$">, <IMG
+ WIDTH="21" HEIGHT="22" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img73.png"
+ ALT="$z'$">, work horizontally in the ring,
+and three similar screws, <IMG
+ WIDTH="13" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img22.png"
+ ALT="$l$">, <IMG
+ WIDTH="23" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img19.png"
+ ALT="$m$">, <IMG
+ WIDTH="18" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img20.png"
+ ALT="$n$">, work vertically through the
+ring at equal intervals. <IMG
+ WIDTH="33" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img74.png"
+ ALT="$AS$"> is the axle of the instrument, <IMG
+ WIDTH="32" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img75.png"
+ ALT="$SS$"> is
+a brass screw working in the upper part of the cone <IMG
+ WIDTH="22" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img3.png"
+ ALT="$C$">, and capable
+of being firmly clamped by means of the nut <IMG
+ WIDTH="15" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img67.png"
+ ALT="$c$">. <IMG
+ WIDTH="22" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img2.png"
+ ALT="$B$"> is a cylindrical
+brass bob, which may be screwed up or down the axis, and fixed in its
+place by the nut <IMG
+ WIDTH="15" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img39.png"
+ ALT="$b$">.
+
+<P>
+The lower extremity of the axle is a fine steel point, finished
+without emery, and afterwards hardened. It runs in a little agate cup
+set in the top of the pillar <IMG
+ WIDTH="21" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img76.png"
+ ALT="$P$">. If any emery had been embedded in
+the steel, the cup would soon be worn out. The upper end of the axle
+has also a steel point by which it may be kept steady while spinning.
+
+<P>
+When the instrument is in use, a coloured disc is attached to the
+upper end of the axle.
+
+<P>
+It will be seen that there are eleven adjustments, nine screws in the
+brass ring, the axle screwing in the cone, and the bob screwing on the
+axle. The advantage of the last two adjustments is, that by them
+large alterations can be made, which are not possible by means of the
+small screws.
+
+<P>
+The first thing to be done with the instrument is, to make the steel
+point at the end of the axle coincide with the centre of gravity of
+the whole. This is done roughly by screwing the axle to the right
+place nearly, and then balancing the instrument on its point, and
+screwing the bob and the horizontal screws till the instrument will
+remain balanced in any position in which it is placed.
+
+<P>
+When this adjustment is carefully made, the rotation of the top has no
+tendency to shake the steel point in the agate cup, however irregular
+the motion may appear to be.
+
+<P>
+The next thing to be done, is to make one of the principal axes of the
+central ellipsoid coincide with the axle of the top.
+
+<P>
+To effect this, we must begin by spinning the top gently about its
+axle, steadying the upper part with the finger at first. If the axle
+is already a principal axis the top will continue to revolve about its
+axle when the finger is removed. If it is not, we observe that the
+top begins to spin about some other axis, and the axle moves away from
+the centre of motion and then back to it again, and so on, alternately
+widening its circles and contracting them.
+
+<P>
+It is impossible to observe this motion successfully, without the aid
+of the coloured disc placed near the upper end of the axis. This disc
+is divided into sectors, and strongly coloured, so that each sector
+may be recognised by its colour when in rapid motion. If the axis
+about which the top is really revolving, falls within this disc, its
+position may be ascertained by the colour of the spot at the centre of
+motion. If the central spot appears red, we know that the invariable
+axis at that instant passes through the red part of the disc.
+
+<P>
+In this way we can trace the motion of the invariable axis in the
+revolving body, and we find that the path which it describes upon the
+disc may be a circle, an ellipse, an hyperbola, or a straight line,
+according to the arrangement of the instrument.
+
+<P>
+In the case in which the invariable axis coincides at first with the
+axle of the top, and returns to it after separating from it for a
+time, its true path is a circle or an ellipse having the axle in its
+<i>circumference</i>. The true principal axis is at the centre of the
+closed curve. It must be made to coincide with the axle by adjusting
+the vertical screws <IMG
+ WIDTH="13" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img22.png"
+ ALT="$l$">, <IMG
+ WIDTH="23" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img19.png"
+ ALT="$m$">, <IMG
+ WIDTH="18" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img20.png"
+ ALT="$n$">.
+
+<P>
+Suppose that the colour of the centre of motion, when farthest from
+the axle, indicated that the axis of rotation passed through the
+sector <IMG
+ WIDTH="19" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img77.png"
+ ALT="$L$">, then the principal axis must also lie in that sector at
+half the distance from the axle.
+
+<P>
+If this principal axis be that of <i>greatest</i> moment of inertia,
+we must <i>raise</i> the screw <IMG
+ WIDTH="13" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img22.png"
+ ALT="$l$"> in order to bring it nearer the
+axle <IMG
+ WIDTH="21" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img1.png"
+ ALT="$A$">. If it be the axis of least moment we must <i>lower</i> the
+screw <IMG
+ WIDTH="13" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img22.png"
+ ALT="$l$">. In this way we may make the principal axis coincide with
+the axle. Let us suppose that the principal axis is that of greatest
+moment of inertia, and that we have made it coincide with the axle of
+the instrument. Let us also suppose that the moments of inertia about
+the other axes are equal, and very little less than that about the
+axle. Let the top be spun about the axle and then receive a
+disturbance which causes it to spin about some other axis. The
+instantaneous axis will not remain at rest either in space or in the
+body. In space it will describe a right cone, completing a revolution
+in somewhat less than the time of revolution of the top. In the body
+it will describe another cone of larger angle in a period which is
+longer as the difference of axes of the body is smaller. The
+invariable axis will be fixed in space, and describe a cone in the
+body.
+
+<P>
+The relation of the different motions may be understood from the
+following illustration. Take a hoop and make it revolve about a stick
+which remains at rest and touches the inside of the hoop. The section
+of the stick represents the path of the instantaneous axis in space,
+the hoop that of the same axis in the body, and the axis of the stick
+the invariable axis. The point of contact represents the pole of the
+instantaneous axis itself, travelling many times round the stick
+before it gets once round the hoop. It is easy to see that the
+direction in which the hoop moves round the stick, so that if the top
+be spinning in the direction <IMG
+ WIDTH="19" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img77.png"
+ ALT="$L$">, <IMG
+ WIDTH="27" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img78.png"
+ ALT="$M$">, <IMG
+ WIDTH="24" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img79.png"
+ ALT="$N$">, the colours will appear in
+the same order.
+
+<P>
+By screwing the bob B up the axle, the difference of the axes of
+inertia may be diminished, and the time of a complete revolution of
+the invariable axis in the body increased. By observing the number of
+revolutions of the top in a complete cycle of colours of the
+invariable axis, we may determine the ratio of the moments of inertia.
+
+<P>
+By screwing the bob up farther, we may make the axle the principal
+axis of <i>least</i> moment of inertia.
+
+<P>
+The motion of the instantaneous axis will then be that of the point of
+contact of the stick with the <i>outside</i> of the hoop rolling on
+it. The order of colours will be <IMG
+ WIDTH="24" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img79.png"
+ ALT="$N$">, <IMG
+ WIDTH="27" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img78.png"
+ ALT="$M$">, <IMG
+ WIDTH="19" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img77.png"
+ ALT="$L$">, if the top be
+spinning in the direction <IMG
+ WIDTH="19" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img77.png"
+ ALT="$L$">, <IMG
+ WIDTH="27" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img78.png"
+ ALT="$M$">, <IMG
+ WIDTH="24" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img79.png"
+ ALT="$N$">, and the more the bob is
+screwed up, the more rapidly will the colours change, till it ceases
+to be possible to make the observations correctly.
+
+<P>
+In calculating the dimensions of the parts of the instrument, it is
+necessary to provide for the exhibition of the instrument with its
+axle either the greatest or the least axis of inertia. The dimensions
+and weights of the parts of the top which I have found most suitable,
+are given in a note at the end of this paper.
+
+<P>
+Now let us make the axes of inertia in the plane of the ring unequal.
+We may do this by screwing the balance screws <IMG
+ WIDTH="17" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img30.png"
+ ALT="$x$"> and <IMG
+ WIDTH="25" HEIGHT="23" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img80.png"
+ ALT="$x^1$"> farther
+from the axle without altering the centre of gravity.
+
+<P>
+Let us suppose the bob <IMG
+ WIDTH="22" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img2.png"
+ ALT="$B$"> screwed up so as to make the axle the axis
+of least inertia. Then the mean axis is parallel to <IMG
+ WIDTH="36" HEIGHT="23" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img81.png"
+ ALT="$xx^1$">, and the
+greatest is at right angles to <IMG
+ WIDTH="36" HEIGHT="23" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img81.png"
+ ALT="$xx^1$"> in the horizontal plane. The
+path of the invariable axis on the disc is no longer a circle but an
+ellipse, concentric with the disc, and having its major axis parallel
+to the mean axis <IMG
+ WIDTH="36" HEIGHT="23" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img81.png"
+ ALT="$xx^1$">.
+
+<P>
+The smaller the difference between the moment of inertia about the
+axle and about the mean axis, the more eccentric the ellipse will be;
+and if, by screwing the bob down, the axle be made the mean axis, the
+path of the invariable axis will be no longer a closed curve, but an
+hyperbola, so that it will depart altogether from the neighbourhood of
+the axle. When the top is in this condition it must be spun gently,
+for it is very difficult to manage it when its motion gets more and
+more eccentric.
+
+<P>
+When the bob is screwed still farther down, the axle becomes the axis
+of greatest inertia, and <IMG
+ WIDTH="36" HEIGHT="23" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img81.png"
+ ALT="$xx^1$"> the least. The major axis of the
+ellipse described by the invariable axis will now be perpendicular to
+<IMG
+ WIDTH="36" HEIGHT="23" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img81.png"
+ ALT="$xx^1$">, and the farther the bob is screwed down, the eccentricity of
+the ellipse will diminish, and the velocity with which it is described
+will increase.
+
+<P>
+I have now described all the phenomena presented by a body revolving
+freely on its centre of gravity. If we wish to trace the motion of
+the invariable axis by means of the coloured sectors, we must make its
+motion very slow compared with that of the top. It is necessary,
+therefore, to make the moments of inertia about the principal axes
+very nearly equal, and in this case a very small change in the
+position of any part of the top will greatly derange the
+<i>position</i> of the principal axis. So that when the top is well
+adjusted, a single turn of one of the screws of the ring is sufficient
+to make the axle no longer a principal axis, and to set the true axis
+at a considerable inclination to the axle of the top.
+
+<P>
+All the adjustments must therefore be most carefully arranged, or we
+may have the whole apparatus deranged by some eccentricity of
+spinning. The method of making the principal axis coincide with the
+axle must be studied and practised, or the first attempt at spinning
+rapidly may end in the destruction of the top, if not the table on
+which it is spun.
+
+<H2>On the Earth&rsquo;s Motion</H2>
+
+<P>
+We must remember that these motions of a body about its centre of
+gravity, are <i>not</i> illustrations of the theory of the precession
+of the Equinoxes. Precession can be illustrated by the apparatus, but
+we must arrange it so that the force of gravity acts the part of the
+attraction of the sun and moon in producing a force tending to alter
+the axis of rotation. This is easily done by bringing the centre of
+gravity of the whole a little below the point on which it spins. The
+theory of such motions is far more easily comprehended than that which
+we have been investigating.
+
+<P>
+But the earth is a body whose principal axes are unequal, and from the
+phenomena of precession we can determine the ratio of the polar and
+equatorial axes of the &ldquo;central ellipsoid;&rdquo; and supposing the earth to
+have been set in motion about any axis except the principal axis, or
+to have had its original axis disturbed in any way, its subsequent
+motion would be that of the top when the bob is a little below the
+critical position.
+
+<P>
+The axis of angular momentum would have an invariable position in
+space, and would travel with respect to the earth round the axis of
+figure with a velocity <!-- MATH
+ $\displaystyle = \omega\frac{C - A}{A}$
+ -->
+<IMG
+ WIDTH="95" HEIGHT="63" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img82.png"
+ ALT="$\displaystyle = \omega\frac{C - A}{A}$"> where
+<IMG
+ WIDTH="19" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img83.png"
+ ALT="$\omega$"> is the sidereal angular velocity of the earth. The apparent
+pole of the earth would travel (with respect to the earth) from west
+to east round the true pole, completing its circuit in <!-- MATH
+ $\displaystyle
+\frac{A}{C - A}$
+ -->
+<IMG
+ WIDTH="63" HEIGHT="63" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img84.png"
+ ALT="$\displaystyle
+\frac{A}{C - A}$"> sidereal days, which appears to be about 325.6 solar
+days.
+
+<P>
+The instantaneous axis would revolve about this axis in space in about
+a day, and would always be in a plane with the true axis of the earth
+and the axis of angular momentum. The effect of such a motion on the
+apparent position of a star would be, that its zenith distance should
+be increased and diminished during a period of 325.6 days. This
+alteration of zenith distance is the same above and below the pole, so
+that the polar distance of the star is unaltered. In fact the method
+of finding the pole of the heavens by observations of stars, gives the
+pole of the <i>invariable axis</i>, which is altered only by external
+forces, such as those of the sun and moon.
+
+<P>
+There is therefore no change in the apparent polar distance of stars
+due to this cause. It is the latitude which varies. The magnitude of
+this variation cannot be determined by theory. The periodic time of
+the variation may be found approximately from the known dynamical
+properties of the earth. The epoch of maximum latitude cannot be
+found except by observation, but it must be later in proportion to the
+east longitude of the observatory.
+
+<P>
+In order to determine the existence of such a variation of latitude, I
+have examined the observations of <i>Polaris</i> with the Greenwich
+Transit Circle in the years 1851-2-3-4. The observations of the upper
+transit during each month were collected, and the mean of each month
+found. The same was done for the lower transits. The difference of
+zenith distance of upper and lower transit is twice the polar distance
+of Polaris, and half the sum gives the co-latitude of Greenwich.
+
+<P>
+In this way I found the apparent co-latitude of Greenwich for each
+month of the four years specified.
+
+<P>
+There appeared a very slight indication of a maximum belonging to the
+set of months,
+
+<P>
+<DIV ALIGN="CENTER">
+<TABLE CELLPADDING=3>
+<TR><TD ALIGN="CENTER">March, 51.</TD>
+<TD ALIGN="CENTER">Feb. 52.</TD>
+<TD ALIGN="CENTER">Dec. 52.</TD>
+<TD ALIGN="CENTER">Nov. 53.</TD>
+<TD ALIGN="CENTER">Sept. 54.</TD>
+</TR>
+</TABLE>
+</DIV>
+
+<P>
+This result, however, is to be regarded as very doubtful, as there did
+not appear to be evidence for any variation exceeding half a second of
+space, and more observations would be required to establish the
+existence of so small a variation at all.
+
+<P>
+I therefore conclude that the earth has been for a long time revolving
+about an axis very near to the axis of figure, if not coinciding with
+it. The cause of this near coincidence is either the original
+softness of the earth, or the present fluidity of its interior. The
+axes of the earth are so nearly equal, that a considerable elevation
+of a tract of country might produce a deviation of the principal axis
+within the limits of observation, and the only cause which would
+restore the uniform motion, would be the action of a fluid which would
+gradually diminish the oscillations of latitude. The permanence of
+latitude essentially depends on the inequality of the earth&rsquo;s axes,
+for if they had been all equal, any alteration of the crust of the
+earth would have produced new principal axes, and the axis of rotation
+would travel about those axes, altering the latitudes of all places,
+and yet not in the least altering the position of the axis of rotation
+among the stars.
+
+<P>
+Perhaps by a more extensive search and analysis of the observations of
+different observatories, the nature of the periodic variation of
+latitude, if it exist, may be determined. I am not aware of any
+calculations having been made to prove its non-existence, although, on
+dynamical grounds, we have every reason to look for some very small
+variation having the periodic time of 325.6 days nearly, a period
+which is clearly distinguished from any other astronomical cycle, and
+therefore easily recognised.
+
+<H2>Note: Dimensions and Weights of the parts of the Dynamical Top.</H2>
+
+<P>
+<TABLE CELLPADDING=3 BORDER="1">
+<TR><TD ALIGN="LEFT" VALIGN="TOP" WIDTH=365>Part</TD>
+<TD ALIGN="CENTER" COLSPAN=2>Weight</TD>
+</TR>
+<TR><TD ALIGN="LEFT" VALIGN="TOP" WIDTH=365>&nbsp;</TD>
+<TD ALIGN="RIGHT">lb.</TD>
+<TD ALIGN="RIGHT">oz.</TD>
+</TR>
+<TR><TD ALIGN="LEFT" COLSPAN=3><B>I. Body of the top--</B></TD>
+</TR>
+<TR><TD ALIGN="LEFT" VALIGN="TOP" WIDTH=365>Mean diameter of ring, 4 inches.</TD>
+<TD ALIGN="RIGHT">&nbsp;</TD>
+<TD ALIGN="RIGHT">&nbsp;</TD>
+</TR>
+<TR><TD ALIGN="LEFT" VALIGN="TOP" WIDTH=365>Section of ring, <IMG
+ WIDTH="17" HEIGHT="42" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img85.png"
+ ALT="$\frac{1}{3}$"> inch square.</TD>
+<TD ALIGN="RIGHT">&nbsp;</TD>
+<TD ALIGN="RIGHT">&nbsp;</TD>
+</TR>
+<TR><TD ALIGN="LEFT" VALIGN="TOP" WIDTH=365>The conical portion rises from the upper and
+inner edge of the ring, a height of <IMG
+ WIDTH="27" HEIGHT="42" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img86.png"
+ ALT="$1\frac{1}{2}$"> inches from the base.</TD>
+<TD ALIGN="RIGHT">&nbsp;</TD>
+<TD ALIGN="RIGHT">&nbsp;</TD>
+</TR>
+<TR><TD ALIGN="LEFT" VALIGN="TOP" WIDTH=365>The whole body of the top weighs</TD>
+<TD ALIGN="RIGHT">1</TD>
+<TD ALIGN="RIGHT">7</TD>
+</TR>
+<TR><TD ALIGN="LEFT" VALIGN="TOP" WIDTH=365>Each of the nine adjusting screws has its screw 1 inch long,
+and the screw and head together weigh 1 ounce.
+ The whole weigh</TD>
+<TD ALIGN="RIGHT">&nbsp;</TD>
+<TD ALIGN="RIGHT">9</TD>
+</TR>
+<TR><TD ALIGN="LEFT" COLSPAN=3><B>II. Axle, &amp;c.--</B></TD>
+</TR>
+<TR><TD ALIGN="LEFT" VALIGN="TOP" WIDTH=365>Length of axle 5 inches, of which <IMG
+ WIDTH="17" HEIGHT="42" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img87.png"
+ ALT="$\frac{1}{2}$"> inch at the
+bottom is occupied by the steel point, <IMG
+ WIDTH="27" HEIGHT="42" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img88.png"
+ ALT="$3\frac{1}{2}$"> inches are brass
+with a good screw turned on it, and the remaining inch is of steel, with
+a sharp point at the top. The whole weighs</TD>
+<TD ALIGN="RIGHT">&nbsp;</TD>
+<TD ALIGN="RIGHT"><IMG
+ WIDTH="27" HEIGHT="42" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img86.png"
+ ALT="$1\frac{1}{2}$"></TD>
+</TR>
+<TR><TD ALIGN="LEFT" VALIGN="TOP" WIDTH=365>The bob <IMG
+ WIDTH="22" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img2.png"
+ ALT="$B$"> has a diameter of 1.4 inches, and a
+thickness of .4. It weighs</TD>
+<TD ALIGN="RIGHT">&nbsp;</TD>
+<TD ALIGN="RIGHT"><IMG
+ WIDTH="27" HEIGHT="42" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img89.png"
+ ALT="$2\frac{3}{4}$"></TD>
+</TR>
+<TR><TD ALIGN="LEFT" VALIGN="TOP" WIDTH=365>The nuts <IMG
+ WIDTH="15" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img39.png"
+ ALT="$b$"> and <IMG
+ WIDTH="15" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
+ SRC="images/img67.png"
+ ALT="$c$">, for clamping the bob and the body
+of the top on the axle, each weigh <IMG
+ WIDTH="17" HEIGHT="42" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img87.png"
+ ALT="$\frac{1}{2}$"> oz.</TD>
+<TD ALIGN="RIGHT">&nbsp;</TD>
+<TD ALIGN="RIGHT">1</TD>
+</TR>
+<TR><TD ALIGN="LEFT" VALIGN="TOP" WIDTH=365><B>Weight of whole top</B></TD>
+<TD ALIGN="RIGHT">2</TD>
+<TD ALIGN="RIGHT"><IMG
+ WIDTH="27" HEIGHT="42" ALIGN="MIDDLE" BORDER="0"
+ SRC="images/img90.png"
+ ALT="$5\frac{1}{4}$"></TD>
+</TR>
+</TABLE>
+
+<P>
+The best arrangement, for general observations, is to have the disc of
+card divided into four quadrants, coloured with vermilion, chrome
+yellow, emerald green, and ultramarine. These are bright colours,
+and, if the vermilion is good, they combine into a grayish tint when
+the rotation is about the axle, and burst into brilliant colours when
+the axis is disturbed. It is useful to have some concentric circles,
+drawn with ink, over the colours, and about 12 radii drawn in strong
+pencil lines. It is easy to distinguish the ink from the pencil
+lines, as they cross the invariable axis, by their want of lustre. In
+this way, the path of the invariable axis may be identified with great
+accuracy, and compared with theory.
+
+<HR>
+
+<P>
+<a name="linknote-1" id="linknote-1"></a> <a href="#linknoteref-1"><big>*</big></a>
+<a name="linknote-2" id="linknote-2"></a> <a href="#linknoteref-2"></a>
+<a name="linknote-3" id="linknote-3"></a> <a href="#linknoteref-3"></a>
+<a name="linknote-4" id="linknote-4"></a> <a href="#linknoteref-4"></a>
+<a name="linknote-5" id="linknote-5"></a> <a href="#linknoteref-5"></a>
+<a name="linknote-6" id="linknote-6"></a> <a href="#linknoteref-6"></a>
+<a name="linknote-7" id="linknote-7"></a> <a href="#linknoteref-7"></a>
+<a name="linknote-8" id="linknote-8"></a> <a href="#linknoteref-8"></a>
+<a name="linknote-10" id="linknote-10"></a> <a href="#linknoteref-10"></a>
+<a name="linknote-11" id="linknote-11"></a> <a href="#linknoteref-11"></a>
+<a name="linknote-12" id="linknote-12"></a> <a href="#linknoteref-12"></a>
+<a name="linknote-13" id="linknote-13"></a> <a href="#linknoteref-13"></a>
+<a name="linknote-14" id="linknote-14"></a> <a href="#linknoteref-14"></a>
+<a name="linknote-15" id="linknote-15"></a> <a href="#linknoteref-15"></a>
+<a name="linknote-16" id="linknote-16"></a> <a href="#linknoteref-16"></a>
+7th May 1857. The paragraphs marked thus have been
+rewritten since the paper was read.
+</P>
+
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