[1] In some cases hydroids have been reared in aquaria from ova of
medusae, but these hydroids have not yet been found in the sea
(Browne [10 a]).
[2] The numbers in square brackets [] refer to the bibliography at
the end of this article; but when the number is preceded by the word
Hydrozoa, it refers to the bibliography at the end of the article
HYDROZOA.
HYDROMETER (Gr. [Greek: hydôr], water, and [Greek: metron], a measure),
an instrument for determining the density of bodies, generally of
fluids, but in some cases of solids. When a body floats in a fluid under
the action of gravity, the weight of the body is equal to that of the
fluid which it displaces (see HYDROMECHANICS). It is upon this principle
that the hydrometer is constructed, and it obviously admits of two modes
of application in the case of fluids: either we may compare the weights
of floating bodies which are capable of displacing the same volume of
different fluids, or we may compare the volumes of the different fluids
which are displaced by the same weight. In the latter case, the
densities of the fluids will be inversely proportional to the volumes
thus displaced.
The hydrometer is said by Synesius Cyreneus in his fifth letter to have
been invented by Hypatia at Alexandria,[1] but appears to have been
neglected until it was reinvented by Robert Boyle, whose "New Essay
Instrument," as described in the _Phil. Trans._ for June 1675, differs
in no essential particular from Nicholson's hydrometer. This instrument
was devised for the purpose of detecting counterfeit coin, especially
guineas and half-guineas. In the first section of the paper (_Phil.
Trans._ No. 115, p. 329) the author refers to a glass instrument
exhibited by himself many years before, and "consisting of a bubble
furnished with a long and slender stem, which was to be put into several
liquors, to compare and estimate their specific gravities." This seems
to be the first reference to the hydrometer in modern times.
In fig. 1 C represents the instrument used for guineas, the circular
plates A representing plates of lead, which are used as ballast when
lighter coins than guineas are examined. B represents "a small glass
instrument for estimating the specific gravities of liquors," an account
of which was promised by Boyle in the following number of the _Phil.
Trans._, but did not appear.
[Illustration: FIG. 1.--Boyle's New Essay Instrument.]
The instrument represented at B (fig. 1), which is copied from Robert
Boyle's sketch in the _Phil. Trans._ for 1675, is generally known as the
common hydrometer. It is usually made of glass, the lower bulb being
loaded with mercury or small shot which serves as ballast, causing the
instrument to float with the stem vertical. The quantity of mercury or
shot inserted depends upon the density of the liquids for which the
hydrometer is to be employed, it being essential that the whole of the
bulb should be immersed in the heaviest liquid for which the instrument
is used, while the length and diameter of the stem must be such that the
hydrometer will float in the lightest liquid for which it is required.
The stem is usually divided into a number of equal parts, the divisions
of the scale being varied in different instruments, according to the
purposes for which they are employed.
Let V denote the volume of the instrument immersed (i.e. of liquid
displaced) when the surface of the liquid in which the hydrometer
floats coincides with the lowest division of the scale, A the area of
the transverse section of the stem, l the length of a scale division,
n the number of divisions on the stem, and W the weight of the
instrument. Suppose the successive divisions of the scale to be
numbered 0, 1, 2 ... n starting with the lowest, and let w0, W1, w2
... w_n be the weights of unit volume of the liquids in which the
hydrometer sinks to the divisions 0, 1, 2 ... n respectively. Then, by
the principle of Archimedes,