194 an empirical study of gyrating bodies. 



sir william Thomson's "gyrostatic" model of the 

 dipping needle. 



In the " Report of British Association," Nature, Sept. 

 25, 1885, page 524, is an account of an instrument named 

 as above, and of certain experiments which Sir William 

 Thomson "believes will be very easily performed, 

 although he has not yet found time to try them." The 

 following describes the instrument with sufficient accu- 

 racy. Suppose a horizontal line to pass through the 

 centre of gravity of the wheel of a gyrostat, perpen- 

 dicular to the axis, and so out through the frame, or 

 ring (if a gyroscope is used), till it projects both ways 

 an inch or so. Suppose, from the ring out, this line to 

 become an infinitely fine and perfectly rigid rod. (These 

 projecting pieces correspond to Sir William's knife 

 edges. See Nature). 



If, now, the instrument be placed so that the plane 

 passing through this line and the axis of the wheel, is 

 horizontal, the two supports being in an east and west 

 line, and, of course, the axle in the meridian ; and if 

 the wheel be made to revolve in the same direction as 

 the earth, the north end will rise till it is parallel to the 

 earth's axis, i. e., it shows the latitude. If carried 

 south, the axis will be level at the equator, and, below 

 that, the other end will rise, till, at the pole, it points 

 vertically upward. 



This, with an absolutely perfect instrument — which 

 means one without friction, — would take place ; but 

 such an instrument is unattainable, and I doubt whether 

 one sufficiently near perfection can ever be made to do 

 it, because the earth's angular motion is so exceed- 

 ingly slow. The principle, however, can easily be illus- 

 trated, the movements all being greatly exaggerated. 

 All we have to do is to place the instrument, a gyrostatic 

 dipping-needle, or compass, or whatever it may be 

 called, on a turn-table (fig. 47). 



178 



