On the Mean Devsity of the Earth. 1 15 



the proportion of these two computed attractions was to be 

 compared with that of the observed effects on the pUuriniets, 

 viz, the lateral deviation bv the hill in comparison with 

 the perpendicular direction of gravity, which comparison of 

 the computed and observed effects would give the ratio of 

 the densities, namelv, of the hill and the earth. 



The magnitude and novelty of these nice calculations, 

 the requisite portion of science and ingenuity for making 

 them with effect, were such as appalled every mind, and 

 every one shrank from the task ; when, at the request of 

 the President and Council of tlteSoeielv, I undertook the per- 

 formance; and after ince;sant labour, during the course of 

 a year, produced the result of the whole, to the entire satis- 

 faction of all the Society. The account of these calcuia.- 

 tions was published in the Philosophical Transactions for 

 the year 1778, and in volume xiv. of my Abridgement of 

 these Transactions; and, though in a very condensed form, 

 occupied no less than a hundred quarto pages in that work, 

 containing only the results of many thousands of intricate 

 calculations. The conclusion from all which was, that the 

 mean density of the whole mass of the earth is nearly 

 double that of the mountain, being to the former in the pro- 

 portion of 9 to 5 ; whence it appears that the density of the 

 earth is about five times that of water, considering the spe- 

 cific gravity of the rock of the hill as between 2*7 and 2*8, 

 as it really is ; and hence also 1 inferred, as a probable de- 

 duction, the necessity of the interior of the earth consislinir, 

 in a great tiieasure, of metallic matter. Thus, then, this 

 grand desideratum was minutely determined, for the first 

 time, by the English nation, and on a large scale of mea' 

 surement and calculation. 



Besides the above computation, no other experiments 

 have been made for the same purpose, exe-ept an attempt 

 made many years afterwards by Mr. Cavendish, and pub- 

 lished in the Philosophical Transactions for the year 1798. 

 'J'his method was by means of two balls of lead fixed to 

 the ends of a straight bar, which was sus|)ended hori2Dntally 

 at its middle point by a fine wire. Then another ball be- 

 ing held in a position opposite to one of the former, its 

 minute attraction was supposed to cause, after a lonu time 

 acting, a very small torsion or twisting of the wire, eup» 

 po^e about a quaner round; from which twisting Mr. C. • 

 dtdueed nearly the same conclusion as by the former grand 

 cxiKTimcnt, viz. that the density of the earth is about U\c 

 tiuK.s the density of water. 



I-1 2 Thus 



