90 Gyrostatic Experiments. 



is called negative. The difference between the motion of the 

 top and of the earth is that, whereas in the former the conical 

 rotation is in the same direction as the axis, in the case of the 

 earth the pole of the -axis turns about the pole of the ecliptic 

 in the opposite direction to that in which the earth revolves 

 about its axis. (By means of a globe and other appliances 

 the lecturer explained the theory of precession and nutation, 

 and their effect upon the polar inclination.) If the earth 

 were truly spherical and homogeneous, or if it were such 

 that the resultant of the attractions exerted on all its parts by 

 any other body should always pass through a definite point in 

 its mass, its diurnal rotation would not be affected by the attrac- 

 tions of any other bodies, and the Pole star would always be the 

 same star. Nutation is a slight oscillatory movement of the 

 earth's axis disturbing the otherwise circular path described by 

 the pole of the earth round that of the ecliptic. In the monthly 

 lunar nutation, the solar semi-annual nutation, and the other 

 movement he had referred to, are comprised the astronomer's 

 gyrostatic problem. There are, however, other problems 

 connected with gyrostatics which are far more difficult to 

 solve. The lecturer next illustrated the rotatory stability of 

 different forms of gyrostats, including prolate, oblate, and 

 ordinary disc and gimbal -formed. He also gave an amusing 

 solution of Columbus's problem how to make an egg stand on 

 end. If the egg is hard-boiled, it is practicable to spin it on 

 end like a top, whereas the viscous fluid in the raw egg pre- 

 vents its being^treated in a similar manner. 



He] believed, if we are ever to solve the difficult problem 

 of the elasticity of matter, it will be by the aid of the phe- 

 nomena of rotation. Accepting the undulatory theory of 

 light, we shall see that Faraday's brilliant discovery demon- 

 strates the gyrostatic influence there. Nothing but the influ- 

 ence due to rotation could produce the effect which, as Faraday 

 discovered, is produced by the magnet upon light passing 

 through;glass>etween the poles of the magnet. Some years 

 ago we were all trying to find some kind of association be- 



