bly the most widely used at the present time.

Tremeschini's pyrometer is based on a different principle, viz., on the
expansion of a thin plate of platinum, which is heated by a mass of metal
previously raised to the temperature of the medium. The exact arrangements
are difficult to describe without the aid of drawings, but the result is
to measure the difference of temperature between the medium to be tested
and the atmosphere at the position of the instrument. The whole apparatus
is simple, compact, and easy to manage, and its indications appear to be
correct at least up to 800 deg.C.

The Trampler pyrometer is based upon the difference in the coefficients of
dilatation for iron and graphite, that of the latter being about
two-thirds that of the former. There is an iron tube containing a stick of
hard graphite. This is placed in the medium to be examined, and both
lengthen under the heat, but the iron the most of the two. At the top of
the stick of graphite is a metal cap carrying a knife-edge, on which rests
a bent lever pressed down upon it by a light spring. A fine chain attached
to the long arm of this lever is wound upon a small pulley; a larger
pulley on the same axis has wound upon it a second chain, which actuates a
third pulley on the axis of the indicating needle. In this way the
relative dilatation of the graphite is sufficiently magnified to be easily
visible.

A somewhat similar instrument is the Gauntlett pyrometer, which is largely
used in the north of England. Here the instrument is partly of iron,
partly of fireclay, and the difference in the expansion of the two
materials is caused to act by a system of springs upon a needle revolving
upon a dial.

The Ducomet pyrometer is on a very different principle, and only
applicable to rough determinations. It consists of a series of rings made
of alloys which have slightly different melting-points. These are strung
upon a rod, which is pushed into the medium to be measured, and are
pressed together by a spiral spring. As soon as any one of the rings
begins to soften under the heat, it is squeezed together by the pressure,
and, as it melts, it is completely squeezed out and disappears. The rod is
then made to rise by the thickness of the melted ring, and a simple
apparatus shows at any moment the number of rings which have melted, and
therefore the temperature which has been attained. This instrument cannot
be used to follow variations of temperature, but