570 PROFESSOR MAXWELL ON A DYNAMICAL TOP. 
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 uni- 
form 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’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 rota- 
tion 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. 
Perhaps by amore 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. 
Nore. 
Dimensions and Weights of the parts of the Dynamical Top. 
I. Body of the top— 
Mean diameter of ring, 4 inches. 
Section of ring, 4 inch square. 
The conical portion rises from the upper and inner edge of the ring, a height 
of 14 inches from the base. 
The whole body of the top weighs “wildlbaintez. 
Each of the nine adjusting screws has its screw 1 inch long, and the screw ' 
and head together weigh 1 ounce. The whole weigh Je 
II. Axle, &¢.— 
Length of axle 5 inches, of which } inch at the bottom is occupied by the steel 
point, 33 inches are brass with a good screw turned on it, and the re- 
maining inch is of steel, with a hep point at the toh The whole 
weighs TR ik, 
The bob B has a diameter of. 1-4 inches, and ¢ a thickness of “4, It weighs ¢ 23 ., 
The nuts 6 and ¢, for clamping the bob and the wes! of the ae on the axle, 
each weigh 4 oz. : : : ; Lei 35 
Weight of whole top 2 Ib. 5} oz. 
The best arrangement, for general observations, is to have the dise 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 revolution 
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 aceu- 
racy, and compared with theory. 
