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2. On a Dynamical Top, for exhibiting the Phenomena of 

 the Motion of a System of invariable form about a Fixed 

 Point ; with some suggestions as to the Earth's Motion. 

 By Professor Clerk Maxwell. 



The top is an instrument similar to that exhibited by the author 

 at the meeting of the British Association in 1856. It differs from 

 it in being of smaller size and entirely of brass, except the ends of 

 the axlo ; and in having six horizontal adjusting screws and three 

 vertical ones, instead of four of each kind. 



It consists of a hollow cone, with a heavy ring round the base, and 

 an axle, terminating in a steel point, screwing through the vertex. 

 In the ring are the nine adjusting screws, and on the axle is a heavy 

 bob, which may be 6xed at any height. 



By means of these adjustments the centre of gravity of the whole 

 is made to coincide with the steel point, and the axle of the top is 

 made one of the principal axes of the central ellipsoid. 



The whole theory of the spinning of such a system about its cen- 

 tre of gravity depends on the form of Poinsot's ellipsoid correspond- 

 ing to the particular arrangement of the screws. The top is intended 

 to exhibit those cases in which the three axes of this ellipsoid are 

 nearly equal. In these cases the instantaneous axis is never far 

 from the normal to the invariable plane, which we may call the in- 

 variable axis. This axis is fixed in space, but not in the body ; for 

 it describes, with respect to the body, a cone of the second order, 

 whose axis is either the greatest or the least of the principal axes of 

 inertia. 



To observe the path of the invariable axis in the rapidly revolving 

 body, we must have the means of recognizing the part of the body 

 through which it passes at any time. For this purpose a disc of 

 card is placed near the upper end of the axle. The four quadrants 

 of this disc are painted red, yellow, green, and blue, and various other 

 marks are added ; so that by observing the colour of the spot which 

 appears the centre of motion, and the diameter of the coloured spot, 

 the position of the invariable axis in the body at any instant may be 

 known, and its path traced out. 



Tliis path is a conic section, whose centre is in the principal axis. 

 If that axis be the greatest or least, it is an ellipse with its major 



