THERMOMETERS.
175
Amongst all these scales, Fahrenheit's preserves its ascendancy ¬
in England. Indeed it possesses many advantages.
Ist. It is the oldest, the others being mere variations, for
the sake of vain glory.
2nd. Its divisions are the smallest, and, therefore, there
is less occasion for fractional expressions.
3d. Its beginning so far below the freezing cold of water,
renders it seldom necessary to have recourse to negative
expressions.
Mortimer's Metallic Thermometer
The usual thermometers can only measure the lower degrees ¬
of temperature, and, being made of glass, are easily
broken by accidents, and as liable to crack of themselves, by
being taken out of a great heat, and too suddenly exposed
to cold. Dr. Cromwell Mortimer considered whether the
property of solids, and especially metals, contracting with
cold, and expanding with heat, might not be applied to the
construction of an instrument capable of measuring all degrees, ¬
even of the greatest cold as well as of the greatest
heat.
Though the alterations in metals are but small, in respect
of those inspirit, or even quicksilver, yet it being found, that
iron becomes one-sixtieth longer when red hot, than when
of the usual temperature, we may conclude that from
Fahrenheit’s cold of 40 below 0, to the greatest heat iron
can bear without melting, a rod three feet long will have
about onc quarter of an inch increase of length; which increase ¬
will be range enough to make all the intérmediate
degrees observable on an instrument.
The instrument, as executed by Mr. Jackson, in the year 1736,
the advice of Mr. Graham and Mr. Ellicot, is represented in figs. 97
and 98.
In fig. 97, a b, is a round rod of steel or brass, a quarter of an inch
thick, and three feet one inch long. When the rod is of brass, three
feet long, the point, a, must be of steel, one inch long, to prevent its
wearing away, or losing its point ; which conical point is made to screw
on and off. C d, are two iron supporters, joined by a flat cross bar at
bottom, d, two inches long, in the middle of which is a point oneeighth ¬
of an inch high, under b, which goes into a hole at the bottom of
the rod, b, and serves to keep the rod in its place at bottom, as the
cross bar, **, having a hole in it through which the rod passes down in
the middle, or about two-thirds up the supporters, and the point, a,
goes into a small hole in the under side of the lever ; all which keep the
upright rod firm and steady in its place. The iron supporters are flat, or
parallel to the front of the machine, from c to x, where they are twisted -
half round, so that the lower parts stand at right angles with the
upper parts.
