256 ON A DYNAMICAL TOP. 



The form of the body of the instrument which I have found most suitable is 

 that of a bell (p. 262, fig. 6). C is a hollow cone of brass, R is a heavy 

 ring cast in the same piece. Six screws, with heavy heads, x, y, z, x', y 1 , z', 

 work horizontally in the ring, and three similar screws, I, m, n, work vertically 

 through the ring at equal intervals. AS is the axle of the instrument, SS is 

 a brass screw working in the upper part of the cone C, and capable of being 

 firmly clamped by means of the nut c. B is a cylindrical brass bob, which may 

 lie screwed up or down the axis, and fixed in its place by the nut b. 



The lower extremity of the axle is a fine steel point, finished without emery, 

 and afterwards hardened. It runs in a little agate cup set in the top of the 

 pillar P. If any emery had been embedded in the steel, the cup would soon 

 be worn out. The upper end of the axle has also a steel point by which it may 

 be kept steady while spinning. 



When the instrument is in use, a coloured disc is attached to the upper 

 end of the axle. 



It will be seen that there are eleven adjustments, nine screws in the brass 

 ring, the axle screwing in the cone, and the bob screwing on the axle. The 

 advantage of the last two adjustments is, that by them large alterations can be 

 made, which are not possible by means of the small screws. 



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 

 tendency 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, 

 steadying 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. If it 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, alternately widening its circles and contracting them. 



