THE INTERIOR OF THE EARTH 295 



pendulum's oscillations ; it is true the mountain's slight elevation 

 one thousand metres did not promise a very marked effect. An 

 experiment of this kind was made in 1821 by the astronomer Carlini, 

 on Mont Cenis, which showed the earth's density to be in the vicinity 

 of the number given by Maskelyne. In 1854 Airy performed an anal- 

 ogous experiment at the bottom of the Harton coal-mine. At a depth 

 of 1,220 feet it was demonstrated that the seconds pendulum advanced 

 in speed two and a quarter seconds per day, and from this it was con- 

 cluded that the mean density of the globe is to that of the surface as 

 2'63 to 1, and, taking the density at the surface to be 2 - 3, that of the 

 globe is 6*1. M. Saigey endeavored to find the density of the globe by 

 the deflection of the plummetdine due to a whole continent's attrac- 

 tion, calculating the theoretic deviation from the vertical at Evaux, a 

 central point of France, and one of the stations of the meridian of 

 Paris. According to Puissant's calculation, there exists between the 

 astronomical and geodetical latitudes of Evaux a difference of about 

 7", which would indicate that the attraction of the southern part of 

 France, i. e., to the south of the latitude of Evaux, exceeds that of the 

 northern portion. Now, with a good orographic chart the average ele- 

 vation of the ground from about Evaux to the Pyrenees, the Alps, and 

 to the neighboring seas can be calculated, and with these data the 

 effect of all the partial attractions that affect the plummet-line at 

 Evaux. M. Saigey has shown that, to account for the discrepancy 

 pointed out by Puissant (who supposes the attraction of the globe to be 

 about 30,000 times greater than that of all France above Evaux), the 

 mean density of the earth must be to that of France alone as 1*7 is to 

 unity. Taking 2*5 for the density of the ground, as compared with 

 water, it gives 4 25 as the density of the globe. 



The researches of Maskelyne, above referred to, may be reduced 

 to a closet experiment : one can weigh the earth in his own room ! 

 This was first done by the illustrious Cavendish. This, the youngest, 

 son of the Duke of Devonshire, who sacrificed his hopes of fortune to 

 his love of science, commenced his career in poverty. " His parents," 

 M. Biot tells us, "seeing that he was good for nothing, treated him 

 with indifference, and gradually became estranged from him. He made 

 amends by becoming one of the first chemists of his time, and, when 

 he had acquired celebrity, one of his uncles, who had been a general 

 abroad, returned at a happy moment to leave him an inheritance of 

 three hundred thousand francs rental. He also left him at his death 

 a fortune of thirty million francs. Cavendish was thus the most 

 wealthy of all the learned, and probably the most learned of all the 

 wealthy." 



Cavendish had received from Hyde-Wollaston an apparatus which 

 he in turn had obtained as a bequest from John Michell, and which 

 was designed to determine the weight of the earth by the attraction 

 exerted by two large globes of lead on two small balls suspended from 



