89 



GYROSTATIC EXPERIMENTS. 



In connection with the popular lecture series, Sir William 

 Thomson gave a lecture, on Tuesday evening, 16th April, in 

 St. George's Hall, on "Gyrostatic Experiments." 



Sir William Thomson said what he wished to bring before 

 them in his lecture that evening bears upon the idea of the ex- 

 planation by motion of the elasticity and other properties of 

 matter. " Gyrostat " is a Greek word meaning a thing which 

 stands up because it spins round. Probably most of us can 

 turn back our memories to a time in our earlier life when we 

 were diligent students of gyrostatics as comprised in the spin- 

 ning top, the humming top, and the teetotum. The subject of 

 rotation — gyrostatics — is perhaps one of the most beaten tracks 

 in the plain of dynamics, and a large part of the theory of 

 dynamical science consists in the theory of rotation. All who 

 have studied mathematics, or dynamics mathematically, must 

 know well its fundamental principles. He first called attention 

 to what is called precessional motion, as exemplified in an or- 

 dinary spinning top. This is the movement by which the axis 

 of the top becomes inclined to the plane upon which it is 

 spinning, giving the peculiar conical undulatory motion which 

 all have seen who are skilled in the use of the toy in question, 

 or of the teetotum. (The lecturer illustrated the movement by 

 means of tops and teetotums spun upon a glass plane.) He 

 then proceeded to refer to the precession of the equinoxes, 

 caused by the combined gyrostatic action of the sun and 

 moon upon the mass of matter accumulated about the earth's 

 equator, which has the effect of making the equinoxes succeed 

 each other in less time than they would otherwise do ; so that 

 it is calculated that the equinoctial points make an entire 

 revolution in about 26,000 years. When the precessional 

 motion is in the same direction as that of the axis it is called 

 positive precession ; when it is in the opposite direction it is 



