200 AN EMPIRICAL STUDY OF GYRATING BODIES. 



Note on Frisi's Laid. 



as follows : WNES is 



N 



The first part of Frisi's Law is demonstrated 1 

 a section of a sphere revolving on the axis, NS, 

 in such a manner that E rises from the paper. 

 Let a force be applied at N tending to depress it 

 below the paper. There will be a line between 

 the two axes that will neither go down with N 

 nor up with E, and that will be the new axis 

 about which E and N will revolve. N' will be 

 the new pole and N' A will be the polar distance 

 of some point, as A. 



The movement of the pole is found to be about 50" in a year, or 5000" 

 in a century ; therefore if Frisi's law is true, the latitude of A and of 

 other places should change constantly. But there is no such change, 

 ergo, there is something wrong about the demonstration. The error is 

 this : No account is taken of the earth's being a solid, subject to the 

 laws of momentum. The earth may be considered as composed of an 

 infinite number of material diameters (more accurately, small cones 

 whose vertices are at the centre) each rigidly in place. Fix the mind 

 upon the one which at the instant the force is applied at N, is perpen- 

 dicular to NWSE, and call it m. When the force depresses N, m will 

 acquire an equal motion towards the north, and, as there is no force to 

 stop it, it will, when it gets to W, still move northward, and tend to 

 tilt the axis NS into the position N S'. What is true of m is true of all 

 these diameters. As A is rigidly connected with m, it will move in the 

 same sense. Its latitude, therefore, will not change, but the old axis 

 will take a new position in space ; but not a new position relatively to 

 points on the surface of the earth. 



1 See Airy's Astronomical Tracts. 

 Also, Encycl. Brit. 8th cd., Vol. xix.. ]>. 1 in. 



184 



