95. The tendency of the earth to draw all bodies towards it is well
illustrated by the useful “line and plummet.” This consists merely of a
string to one end of which a leaden weight is attached. The string when
at rest hangs vertically; if the weight be drawn to one side, it will,
when released, swing backwards and forwards, until it finally settles
again in the vertical; the reason is that the weight always tries to
get as near the earth as it can, and this is accomplished when the
string hangs vertically downwards.
96. The surface of water in equilibrium is a horizontal plane; that is
also a consequence of gravity. All the particles of water try to get as
near the earth as possible, and therefore if any portion of the water
were higher than the rest, it would immediately spread, as by doing so
it could get lower.
97. Hence the surface of a fluid at rest enables us to find a perfectly
horizontal plane, while the plummet gives us a perfectly vertical
line: both these consequences of gravity are of the utmost practical
importance.
98. The spirit-level is another common and very useful instrument which
depends on gravity. It consists of a glass tube slightly curved, with
its convex surface upwards, and attached to a stand with a flat base.
This tube is nearly filled with spirit, but a bubble of air is allowed
to remain. The tube is permanently adjusted so that, when the plate
is laid on a perfectly horizontal surface, the bubble will stand in
the middle: accordingly the position of the bubble gives a means of
ascertaining whether a surface is level.