th there is a very large proportion of iron. An
iron earth would weigh about seven times as much as an equal globe of
water. We are thus led to see that the earth's weight must be probably
more than three, and probably less than seven, times an equal globe of
water; and hence, in fixing the density between five and six, Newton
adopted a result plausible at the moment, and since shown to be probably
correct. Several methods have been proposed by which this important
question can be solved with accuracy. Of all these methods we shall here
only describe one, because it illustrates, in a very remarkable manner,
the law of universal gravitation.

In the chapter on Gravitation it was pointed out that the intensity of
this force between two masses of moderate dimensions was extremely
minute, and the difficulty in weighing the earth arises from this cause.
The practical application of the process is encumbered by multitudinous
details, which it will be unnecessary for us to consider at present. The
principle of the process is simple enough. To give definiteness to our
description, let us conceive a large globe about two feet in diameter;
and as it is desirable for this globe to be as heavy as possible, let us
suppose it to be made of lead. A small globe brought near the large one
is attracted by the force of gravitation. The amount of this attraction
is extremely small, but, nevertheless, it can be measured by a refined
process which renders extremely small forces sensible. The intensity of
the attraction depends both on the masses of the globes and on their
distance apart, as well as on the force of gravitation. We can also
readily measure the attraction of the earth upon the small globe. This
is, in fact, nothing more nor less than the weight of the small globe in
the ordinary acceptation of the word. We can thus compare the
attraction exerted by the leaden globe with the attraction exerted by
the earth.

If the centre of the earth and the centre of the leaden globe were at
the same distance from the attracted body, then the intensity of their
attractions would give at once the ratio of their masses by simple
proportion. In this case, however, matters are not so simple: the leaden
ball is only distant by a few inches from the attracted ball, while the
centre of the earth's attraction is nearly 4,000 miles away at the
centre of the earth. Allowance has to be made for this difference, and
the attraction of the leaden sphere ha