196 AN EMPIRICAL STUDY OF GYRATING BODIES. 



west, as in the left hand side of tig. 47. It will, with 

 equal readiness, rise to a vertical, and so far the results 

 are the same whether the axis pointed, when horizontal, 

 east and west or north and south. This, however, would 

 be in high latitudes, in fact, quite near the pole. But 

 as our instrument — we are supposing it to be absolutely 

 perfect — is carried from the pole, the tendency to rise 

 becomes less and less, until, at the equator, it becomes 

 null, and the axis may be placed at any angle, and there 

 it will remain. Turn the instrument around 90°, and 

 the axis will pass from any position to a horizontal one. 

 Turn the instrument back 90°, and it again becomes in- 

 different. 



The same rationale, mutatis mutandis, applies here. 



Sir William Thomson says, that with the axis of the 

 rjerfect "gyrostat" pointing north, it will oscillate like 

 a dipping-needle if it be drawn out of its position. This 

 is true only when the centre of gravity is below the 

 line of the supporting edges. 1 In this case the vibration 

 or oscillation, is that of a pendulum, the rotation of the 

 earth being so slow as practically to produce no effect. 

 But if the turn-table is in motion, the axis of the gyro- 

 stat will become parallel to the axis of the former ; but 

 will not oscillate in the least. 



With a perfect instrument, without friction and with 

 the centre of gravity in the line of the support, the axle- 

 will become, as has been said, parallel to the earth's 

 axis ; but there will be no oscillation. It will move into 

 position and then stop. 



Some interesting experiments can be performed with 

 this instrument. 



Block the turn-table so as to prevent rotation or un- 

 steadiness. Set the wheel to revolving rapidly on its 

 axis. Then set it to revolving on the axis whose line is 



1 It is important to remember that in the " compass " there is a absolutely no freedom of 

 motion in azimuth. 



180 



