CHAPTER XV.

HYDROSTATICS.
The Hydrometer.—The hydrometer is an instrument for finding the
relative density or specific gravity of liquids. The name is derived
from two Greek words, Jiydor (water) and metro (measure).
The Principle of the hydrometer is based on the law of Archimedes,
which states that "all floating bodies displace a quantity of liquid equal
to their own. weight/*

Construction.—The marine hydrometer consists of a
glass cylinder with a bulb on its lower end containing
mercury or small shot to act as ballast to keep the instru-
ment upright when floating. The cylinder supports a
stem which carries the scale. The volume of the
cylinder is about 25 times the volume of the stem.
Glass is the best material for use in salt water. If
the hydrometer were made of metal it might become
corroded by the action of the salt, and its indications
would be erroneous. Even when made of glass it must
be kept scrupulously clean and all smears or greasiness
removed by wiping the instrument with a clean soft cloth
beiore and after use.
The Scale is graduated irom 0 at the top to 25 or 40
at the bottom. The simplest form of hydrometer, but
one inconveniently long, would consist of a rod of
uniform bore, and the longer and thinner the rod the
greater would be its sensibility in detecting differences
of density as measured by the scale, the graduations of
which would be proportional to the volume of the rod
and inversely proportional to its cross sectional area.
The marine hydrometer expresses equal differences of
density for successive divisions, hence the reason why
the graduations become closer towards the lower end
of the scale.
The scale indicates the specific gravity of the E<juid in which the
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