650 



Scientific Proceedings, Royal Dublin Society. 



attraction of gravitation, and, therefore, with some corrections for 

 local irregularities, to the distance from the centre of the earth, we 

 have a means of ascertaining approximately the ocean-level at 

 different points ; and numerous observations of the length of the 

 pendulum have been made both on continents and in islands. 

 These have been collected by Airy in his elaborate article on " The 

 Figure of the Earth," published in the Encyclopaedia Metropolitan, 

 from which I extract a few special examples. It will be observed 

 that the length is mainly determined by the latitude. The obser- 

 vations were originally made by Sabine, Biot, and others, and are 

 considered by Airy as "first-rate observations" in contradistinc- 

 tion to others, which he classes as " second-rate." 1 



The name " California " in the above Table is clearly a mistake, 

 if we go by the latitude, which is the more reliable of the two. 

 California, as at present represented on our maps, does not reach 

 so far south as 21° 80' N. latitude. The cause of the decrease in 

 the length of the second's pendulum, as we proceed from the poles 

 towards the equator, is twofold — first, on account of the in- 



1 Airy, "Figure of the Earth," Encyclopedia Metropolitana, vol. v., p. 229. 



2 Island in Gulf of Guinea. 



