158 
WEIGHING THE EARTH WITH A CHEMICAL BALANCE. 
as heavy as an equal sized globe of water. In 1774 Dr. 
Maskelyne compared the attraction of the mountain of 
Schiehallien, in Perthshire, upon a leaden weight suspended 
by a plumb-line, with the attraction of the earth for the same 
weight, and obtained a result for the earth’s density of 4f 
times that of water. In such cases the difficulty of correctly 
estimating the mass of the mountain is very great. In 1854 
(Sir) G. B. Airy obtained a result of 6J from his experiments 
made by swinging a pendulum at the top and at the bottom 
of the shaft of Harton Colliery, near South Shields. The 
famous experiment made by Cavendish in 1798, and repeated 
by Reich in 1887, and (with immense care) by Bailey in 
1842, depended upon the perturbations produced in the 
vibrations of two small balls (fixed one at each end of a light 
rod suspended by a wire from its centre), when large balls of 
lead are brought near to the opposite sides of the small balls. 
The results obtained by this method vary from a little below 
to a little above 5J. 
The latest experiment, and that by a new method, having 
for its object the determination of the density of the earth, 
was devised and carried out by (Prof.) J. H. Poynting, in 
1878, at the Owens College, Manchester. A small metal 
ball, weighing, say, one pound, is attached to the end of one 
arm of a chemical balance of very great sensitiveness, and 
the earth’s pull upon the ball is counterbalanced with the 
utmost nicety by placing weights in the pan suspended from 
the end of the other arm. A heavy mass of metal (in the 
actual experiment a ball of lead weighing 840 lbs. was em¬ 
ployed) is then placed immediately underneath the small ball. 
This small ball is then attracted both by the earth and by 
the large leaden mass, and its weight is consequently in¬ 
creased ; the actual increase observed being one forty-five- 
millionth ! Small as this quantity may seem, it w T as found 
to be quite measurable. In this way we are able to find out 
how strongly the small ball would be attracted by a mass of 
lead the size of the earth, and it is found that it would be 
attracted about twice as strongly as it actually is by the 
earth. But we know the density of lead is 11^-, therefore the 
density of the earth must be one-half of this, or rather more 
than 5J. The precise result obtained by Prof. Poynting was 
5’GOO. We believe that Prof. Poynting is about to repeat 
this very striking and original experiment, with certain 
modifications and improvements in small details, in a base¬ 
ment room (the balance must rest upon the ground to secure 
perfect stability) of the Mason College, Birmingham. 
W. J. H. 
