146 GALILEO AND HIS JUDGES. 



to confirm the truth of the Earth's diurnal revolu- 

 tion. 



Before the close of the seventeenth century it was 

 observed that a diminution of gravity occurred at, 

 and near, the equator. This was proved by the 

 vibration of the pendulum, an experiment associated 

 chiefly with the name of Eicher ; and it has, if I 

 mistake not, been since then carefully tested by 

 spring balances. This phenomenon is owing partly 

 to the spheroidal figure of the Earth itself the 

 result of the rotation on the axis but principally to 

 the centrifugal tendency being greater at the equator, 

 from the higher velocity of rotation. 



I have already alluded to the trade winds, and the 

 argument to be drawn from them, which I think a 

 sound and strong one ; but I need not dwell on it 

 further. 



It is, however, well worth remembering that in 

 our own day another proof has been given, which 

 has been generally allowed to be an important one. 

 It is the result of an experiment of Foucault, and 

 is simply this : if a pendulum, with a heavy weight 

 attached to it, be made to oscillate in a plane due 

 north and south, say in the latitude of Paris, the 

 pendulum, after a time, and supposing it to continue 

 in movement long enough for the purpose of ob- 

 servation, will oscillate in a direction slightly north- 

 east and south-west. Now the pendulum moves 

 naturally always in the same direction, backwards 

 and forwards, as originally started, and if the Earth 



