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 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. 



I. Body of the top — 



Mean diameter of ring, 4 inches. 



Section of ring, ^ inch square. 



The conical portion rises from the upper and inner edge of the ring, a height 



of li inches from the base. 

 The whole body of the top weighs . . . . 1 lb. 7 oz. 



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 1 inch at the bottom is occupied by the steel 

 point, 3^ inches are brass with a good screw turned on it, and the re- 

 maining inch is of steel, with a sharp point at the top. The whole 

 weighs . . . . .... li „ 



The bob B has a diameter of 1*4 inches, and a thickness of *4. It weighs . 1\ ., 



The nuts b and c, for clamping the bob and the body of the top on the axle, 



each weigh ^ oz. . . . . . . 1 „ 



Weight of whole top 2 lb. 5^ oz. 



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 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 accu- 

 racy, and compared with theory. 



