lo determine the Density of the Earth. 361 
that the mean specific gravity of the earth would be about five 
times that of water. The only objection to this admirable ex- 
periment is, that the form of the country near the mountain is 
very irregular, and it is difficult to say how much of the 12 
seconds is or is not really due to Schehallien. 
The second class is what may be called a cabinet experiment, 
possessing the advantage of being extremely manageable, and 
the disadvantage of being exceedingly delicate, and liable to de- 
rangement by forces so trifling that they could with difficulty be 
avoided. ‘'T'wo small balls upon a light horizontal rod were sus- 
pended by a wire, or two wires, forming a torsion balance, and 
two large leaden balls were brought vear to attract the small 
_ balls from the quiescent position. We could make a calculation 
of how far the great balls would attract the little ones, if they 
Were as dense as the general mass of the earth; and comparing 
this with the distance to which the leaden balls really do attract 
them, we find the proportion of the density of the earth to the 
density of lead. The peculiar difficulty aud doubt of the results 
in this experiment depend on the liability to disturbances from 
other causes than the attraction of the leaden balls, especially the 
currents of air produced by the approach of bodies of a different 
temperature; and after all the cautions of Cavendish, Reich, and 
Bailey, in their successive attempts, it seems not impossible that 
the phenomena observed may have been produced in part by the 
temperature of the great balls as well as their attraction. 
These considerations induced Mr. Airy, in 1826, to contem- 
plate a third class of experiments, namely, the determination of 
the difference of gravity at the top and the bottom of a deep 
mine, by pendulum experiments. Supposing the difference of 
gravity found, its application to the determination of density (in 
the simplest case) was thus explained. Conceive a spheroid con- 
centric with the external spheroid of the earth to pass through 
the lower station in the mine. It is easily shown that the attrac- 
tion of the shell included between these produces no effect what- 
ever at the lower station, but produces the same effect at the 
Upper station as if all its matter were collected at the earth’s 
