50 



these gives us the effect of the attraction of that mountain, as 

 compared with the attraction of the earth. There is a mountain 

 at a certain distance, and this great earth at another certain 

 distance : by due computation from that, with the attractions 

 above-mentioned, we find the proportion of the mass of matter 

 in that mountain to the mass of matter in the whole earth. 

 Then as we also know the magnitude of the mountain and the 

 magnitude of the earth, we can compare the density of the moun- 

 tain with the density of the earth. This was gone through with 

 extreme care, and the density of the earth was ascertained to be 

 four-and-a-half times the density of water ; the earth therefore is 

 more dense towards the centre than at the outside. 



The next experiment that was made was almost free from 

 objection. It is what is known as the Cavendish experiment. It 

 was a cabinet experiment entirely. Here is a rod with two light 

 balls upon it, suspended by a fine wire, which is the most delicate 

 suspender we have. Then two huge masses of lead were placed 

 one on one side of one ball, and one on the other side of the 

 other ball, and they tended to pull the rod round by attracting the 

 small balls. Then the experiment was varied by placing the 

 masses of lead each on the opposite side of its small ball ; and in 

 that manner the circumstances were changed so as to show how 

 much the great balls of lead attracted these little balls. By a 

 series of calculations, of which I cannot pretend to give you a 

 precise idea, the density of the earth was ascertained, and that 

 gave a greater result than before, namely — that the average density 

 of the earth was about five-and-a-half times that of water. 



The third experiment was one I made myself in the Harton 

 Colliery, near South Shields. It was judging how much the 

 force of gravity was altered by going to a great depth, that force of 

 gravity at the top and bottom being ascertained by swinging a 

 pendulum. At a depth of twelve hundred feet the pendulum 

 swung two seconds more in a day. That gave the average density 

 of the earth as a little more than six times that of water. Every 

 experiment is open to some objection ; but I believe the best 

 calculation is the one founded upon the Cavendish experiment, 



