566 PROFESSOR MAXWELL ON A DYNAMICAL TOP. 
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. 
When this adjustment is carefully made, the rotation of the top has no ten- 
dency to shake the steel point in the agate cup, however irregular the motion may 
appear to be. ' 
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. 
To effect this, we must begin by spinning the top gently about its axle, steady- 
ing 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. Ifit 
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, al- 
ternately widening its circles and contracting them. 
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. 
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 acircle, 
an ellipse, an hyperbola, or a straight line, according to the arrangement of the 
instrument. 
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 isa 
circle or an ellipse having the axle in its circumference. 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 /, m, n. 
Suppose that the colour of the centre of motion, when farthest from the axle, 
indicated that the axis of rotation passed through the sector L, then the principal 
axis must also lie in that sector at half the distance from the axle. 
If this principal axis be that of greatest moment of inertia, we must raise the 
screw / in order to bring it nearer the axle A. If it be the axis of least moment 
we must /ower the screw /. 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, 
