THE INTERIOR OF THE EARTH. 297 



plisked this work. Their experiments, commenced in 1870, have been 

 the subject of various interesting communications to the Academy of 

 Sciences. Their apparatus are deposited in the vaults of the Poly- 

 technic School. They are much smaller than those of Cavendish and 

 Baily, for, as Messrs. Cornu and Bailie have remarked, there is an 

 advantage gained in these experiments by reducing the dimensions of 

 the apparatus. The attracting mass, formed of mercury contained in 

 two hollow spheres of bronze, 0*12 metre in diameter, weigh twelve 

 kilogrammes. By transpiration the mercury can be made to pass 

 from one sphere to the other, thus doubling the effect of the attrac- 

 tion, and this change is effected without shock or disturbance.* The 

 lever of the torsion-balance is a little tube of aluminium, 0*50 metre 

 in length, cai'rying at each end a ball of copper weighing one hun- 

 dred and nine grammes. A flat mirror fixed in the middle reflects 

 the divisions of a horizontal scale five or six metres distant, and the 

 slightest movement of the lever is thus revealed by a displacement of 

 the scale divisions. The time of a double oscillation of the lever is 

 about seven minutes. The phases of these oscillations are registered 

 by electricity. A great merit of these researches consists in the 

 opportunity they afford for a thorough study of all the causes of per- 

 turbation that can introduce error into such experiments. The defi- 

 nite result can be accepted with confidence. The figure thus far 

 obtained is 5*56. It maybe added that Messrs. Cornu and Bailie have 

 discovered the cause of the too large number given by Baily. In cor- 

 recting the ei'rors of system in his experiments, it is probable that a 

 slightly different number will be obtained 5*55. To sum up, the 

 earth's mean density thus appears to be five and a half times that of 

 water, and the density at the surface is less than half that of the 

 interior, or about 2 5. Consequently there must be in the interior 

 heavy masses whose excess of density compensates for the lack thereof 

 in the rocks at the surface. This need not be surprising, for the heavy 

 pressure sustained by the deeper strata must naturally increase the 

 density. But what is the law governing this increase of density from 

 surface to center ? Legendre formulated a simple law, adopted also by 

 Laplace, according to which the surface density is 2*5, at the middle 

 of the radius 8*5, and at the center 11*3, the mean being taken as 5 - 5. 

 A different law, to which M. Edouard Roche arrived by theoretic con- 

 siderations, gives a surface density of 2*1, a mid-radius density of 8*5, 

 and 106 at the center. This agreement of results deduced from three 

 different hypotheses shows that the decision of the question is nar- 

 rowed to small dimensions. Adopting M. Roche's conclusions as the 

 most probable, it can be said that the mean density of the earth is 

 about double that of its surface, and that the density of the center is 

 double that of its mid-radius. The central strata or masses have a 

 density approximating to that of lead. 



* In the later experiments the number of the spheres was doubled. 



