G. B. Airy on Pendulum experiments, etc. 359 
Art. XXXVII.—On the Pendulum experiments lately made in 
the Harton Colliery, for ascertaining the mean density of the 
Earth; by G. B. Airy, Esq., F. R. S., Astronomer Royal.* 
HE speaker commenced with remarking that the bearing of 
the experiments, of which he was about to give a notice, was 
not limited to their ostensible object, but that it applied to all the 
bodies of the solar system. The professed object of the experi- 
ments was to obtain a measure of the density of the earth, and 
therefore of the mass of the earth (its dimensions being known); 
but the ordinary data of astronomy, taken in conjunction with 
the laws of gravitation, give the proportions of the mass of the 
earth to the masses of the sun and the principal planets; and 
thus the determination of the absolute mass of the earth would 
at once give determinations of the absolute masses of the sun 
and planets. To show how this proportion is ascertained, it is 
ouly necessary to remark, that a planet, if no force acted on it, 
would move ina straight line; that, therefore, if we compute 
Speaker did not attempt to enter: he remarked only that they 
rest upon very complicated chains of reasoning, but of the most 
Certain kind. His only object was to show that the proportion 
of the masses of all bodies, which have planets or satellites re- 
* Proc, Roy. Inst. of Great Britain, Part V, p. 17. 
