Page:Popular Science Monthly Volume 17.djvu/309

Rh 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 analogous 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 concluded 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 attraction, 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 elevation 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