sbct. iv.] DISSERTATION SECONB. 169 



two minutes and a half a-day nearly- This created great 

 astonishment in France, especially after the accuracy of it 

 was confirmed by the observations of Varin and Deshayes, 

 who, some years afterwards, visited different places on the 

 coast of Africa and America, near the line, and found the 

 necessity of shortening the pendulum, to make it vibrate 

 seconds in those latitudes. The first explanation of this 

 remarkable phenomenon was given by Newton, in the third 

 book of his Principia, published in 1 687, where it is de- 

 duced as a necessary consequence of the earth's rotation 

 on its axis, and of the centrifugal force thence arising. 

 That force changes both the direction and the intensity of 

 gravity, giving to the earth an oblate spheroidal figure, more 

 elevated at the equator than the poles, and making bodies 

 fall, and pendulums vibrate, more slowly in low than in 

 high latitudes. 



This solution, however, did not, any more than the book 

 in which it was contained, make its way very readily into 

 France. The first explanation of the retardation of the 

 pendulum, which was received there, was given by Huy- 

 gens in 1690. Huygens deduced it also from a centrifugal 

 force, arising from the earth's rotation, and the view which 

 he took was simpler, though much less accurate than that 

 of Newton. It had, indeed, the simplicity which often 

 arises from neglecting one of the essential conditions of a 

 problem ; but it was nevertheless ingenious, and involved 

 a very accurate knowledge of the nature of centrifugal 

 force. I am thus brought to touch on a subject which be- 

 longs properly to the second part of this Dissertation, for 

 which the fuller discussion of it must of course be re- 

 served. 



