The Locomotive - Lighthouse Survival Blog

1901] THE LOCOMOTIVE. 75
the intensity of gravitative attraction varies with mass and distance. It does not throw
any light at all upon the reason why one body attracts another one.
The main difficulty in the way of learning something about the real nature of grav-
itation is, that there does not appear to be any starting jjoint from which we can begin
our investigations. If we could only find some little peculiarity or apparent irregularity
of gravitative action, we should have a foot-hold at once, and we could then begin a
series of experimental researches into the nature of this peculiarity or irregularity, which
might end in our learning something about the nature of gravitative action.
Wherever we look for such a starting point, however, we find nothing in the least
encouraging. For example, most of the properties of bodies vary according to the
quantity of heat they contain. It is natural to inquire, therefore, whether this may not
be the case with the earth's attraction for a body. There could be no good reason
assigned in advance, for example, why the earth should attract a given mass of ice with
precisely the same force as it would exert on the same mass after it has been melted into
water by the addition of heat. Yet Rumford, who investigated this particular point
very carefully a hundred years ago, says that he could find no difference Avhatever. He
found that the earth attracted the ice and the equivalent water with precisely the same
force ; and no subsequent experimenter has found any other result.
If we inquire whether the earth attracts a body with precisely the same force,
whether the body is hot or cold, we meet with the same unpromising result. Some
experimenters who have worked at this particular problem (among them the writer him-
self) have fancied that there is a slight difference in the weight of a body when it is
heated; but it is not easy to weigh a hot body with precision, and it has not yet been
proved that the slight differences that have been observed are not due to errors in the
weighings.
Again, it might be supposed that two chemical elements might be attracted by the
earth with a different force when they are separate, than would be exerted upon them
when they are chemically combined, so as to form a third substance. Careful experiments
have indeed indicated a slight difference here also; but if such a difference
really exists, it is so small that experimenters have been unable, as yet, to prove that
it is real, and not due, as before, to errors in the weighing, or to slight losses in the
chemical manipulation.
Certain kinds of crystals have different properties in different directions. That is,
their tensile strength and shearing strength vary according to the direction in which the
force is applied; when warmed they expand differently in different directions; light
passes through them with different speeds in different directions; and so on. It is
natural to inquire whether the earth attracts these crystals with precisely the same force,
no matter what the direction in which the pull acts; and when the experiment is tried,
it is found that the direction of the pull makes absolutely no difference at all. The earth
attracts them with precisely the same force, in all positions.
There is one other most amazing thing about gravitation that we must notice; and
that is, that two given masses of matter will attract each other with precisely the same
force, whether the space between them is empty, or filled with other matter. This may
be expressed otherwise by saying that matter appears to be absolutely transparent to
gravitative action. The best way to test this point is by calculating what the motions
of the planets would be, on the supposition that matter is more or less opaque to gravi-
tation, so that a given ton of the sun's mass, on the remote side of that luminary, does
not attract the proposed planet with the same force that it would exert if the interven-
ing body of the sun were removed. We shall find, if we carry out this calculation, that