CH. xxxi. THE SCHEHALL1ON EXPERIMENT. 289 



exactly how much the lines D F will slope towards each 

 other when no mountain is between them. This measure- 

 ment being known, Maskelyne then made two observations, 

 one on each side of Schehallion, and found that in this case 

 the inclination, instead of being from D to F on each side, 

 was from E to F,. because the mountain drew the lead to- 

 wards itself on either side. So the deflection E F D, through 

 which the plumb-line was drawn from the perpendicular, 

 showed the difference between the pull of the whole earth 

 and the pull of the mountain. 



Then Dr. Hutton, the celebrated geologist, set to work 

 to find out the size and weight of Schehallion. This he did 

 by surveying it and measuring it in every direction, and 

 then taking pieces of the different rocks it contained and 

 weighing them carefully. When this was done, it was found 

 that the mountain pulled half as strongly in comparison to 

 its size as the earth did for its size. This showed that the 

 materials in the mountain were half as heavy as the average 

 of those in the earth generally, and as they were also about 

 2\ times as heavy, bulk for bulk, as water, it was proved that 

 the whole globe is about five times heavier than it would be 

 if it was made entirely of that fluid. 



This calculation must be very near the truth, for the 

 chemist Cavendish obtained nearly the same result from 

 quite a different experiment made with suspended balls. 

 This, which is called the 'Cavendish experiment,' is too 

 difficult to explain here. In our own times, Francis Baily, 

 Sir Henry James, Sir Edward Sabine, and others, have re- 

 peated these observations, and found them to be correct. 



Summary of the Science of the Eighteenth Century. 

 This sketch of the advance of astronomy brings us to the 

 end of the science of the eighteenth century ; for although 

 the greater number of the eminent scientific men of our day 



