2.S8 



EIGHTEENTH CENTURY. 



PT. III. 



the end of it, will not point straight to the centre of the 

 earth when it is held near a mountain, because the moun- 

 tain attracts the lead and draws it slightly towards itself. 

 Therefore, if the size and weight of the mountain were 

 known, and it were also known how great its pull is com- 

 pared to the pull of the whole earth, this would enable a 



mathematician to calculate 

 the weight of our entire globe. 

 A man named Bouguer 

 was the first to make this ex- 

 periment near a high moun- 

 tain in Peru in 1738, but he 

 succeeded very imperfectly, 

 and in 1772 Maskelyne pro- 

 posed to the Royal Society 

 to repeat the observation. 



Accordingly, he went in 

 1 7 74, to a very high mountain 

 called Schehallion, near Loch 

 Experiment for estimating Tay, in Perthshire, and there 



measured the mc li nation 



the Density of the Earth (Herschel). 

 A B, Surface of the earth. D, c, D, Ancle , .. , , , , . 



formed by the two plumb-lines point- r sl P e Of tn e plumb-line OH 



eac h S [^ Q Q f t h e mountain. 



ing to the centre of the earth. E, G, 

 E, Angle formed by the two plumb- 

 lines when drawn aside by the mouu- You will remember that, ac- 



cording to the theory of 



gravitation, the lead at the end of the line would 

 point straight to the centre of the earth c if the mountain 

 did not disturb it; 1 and if the plumb-line is taken to two 

 places a certain distance apart and its inclination measured 

 by means of one of the stars overhead, it is easy to find o.it 



1 This is not strictly true, on account of bulge at the equator and 

 flattening of the poles ; but the discrepancy is of no importance to the 

 argument. 



