278 EIGHTEENTH CENTURY. pt. iii. 



in weight, and we do not know what the middle of the globe 

 is made of, this plan is not possible. We know, however, 

 that every atom of matter has the power of attraction, so 

 that if we could find out how much attraction our earth pos- 

 sesses, by comparing it with the attraction of some other 

 body which we can weigh, then we could arrive at the weight 

 of the earth. 



Now Newton, in his * Principia,' had pointed out that a 

 plumb-line, that is, a piece of string with a weight of lead at 

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

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

 attracts the lead and draws it slightly towards itself. There- 

 fore, if the size and weight of the mountain were known, and 

 it were also known how great its pull is compared to the 

 pull of the whole earth, this would enable a mathematician 

 to calculate the weight of our entire globe. 



A man named Bougier was the first to make this experi- 

 ment near a high mountain in Peru in 1738, but he suc- 

 ceeded very imperfectly, and in 1772 Maskelyne proposed 

 to the Royal Society to repeat the observation. Accord- 

 ingly, he went in 1774 to a very high mountain called Sche- 

 hallien, near Loch Tay, in Perthshire, and there he measured 

 the inclination or slope of the plumb-line on each side of 

 the mountain. You will remember that, according 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 ; * 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 out 



• 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. 



