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’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.
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.
Note: Dimensions and Weights of the parts of the Dynamical Top.
| Part | Weight | |
| lb. | oz. | |
| I. Body of the top-- | ||
| Mean diameter of ring, 4 inches. | ||
Section of ring,  inch square. | ||
The conical portion rises from the upper andinner edge of the ring, a height of  inches from the base. | ||
| The whole body of the top weighs | 1 | 7 |
| Each of the nine adjusting screws has its screw 1 inch long,and the screw and head together weigh 1 ounce. The whole weigh | 9 | |
| II. Axle, &c.-- | ||
Length of axle 5 inches, of which  inch at thebottom is occupied by the steel point,  inches are brasswith a good screw turned on it, and the remaining inch is of steel, witha sharp point at the top. The whole weighs | | |
The bob  has a diameter of 1.4 inches, and athickness of .4. It weighs |  | |
The nuts  and  , for clamping the bob and the bodyof the top on the axle, each weigh oz. | 1 | |
| Weight of whole top | 2 |  |
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.
[*] 7th May 1857. The paragraphs marked thus have been rewritten since the paper was read.