indicates clearly the
moment when a particular temperature is attained. It is of course entirely
dependent on the accuracy with which the melting-points of the various
alloys have been fixed.

Yet another principle is involved in the instrument called the
thalpotasimeter, which may be used either with ether, water, or mercury.
It is based on the principle that the pressure of any saturated vapor
corresponds to its temperature. The instrument consists of a tube of metal
partly filled with liquid, which is exposed to the medium which is to be
measured. A metallic pressure gauge is connected with the tube, and
indicates the pressure existing within it at any moment. By graduating the
face of the gauge when the instrument is at known temperatures, the
temperature can be read off directly from the position of the needle. From
100 deg. to 220 deg.F. ether is the liquid used, from thence to 680 deg. it is water,
and above the latter temperature mercury is employed.

Another class of pyrometers having great promise in the future is based on
what may be called the "water-current" principle. Here the temperature is
determined by noting the amount of heat communicated to a known current of
water circulating in the medium to be observed. The idea, which was due to
M. De Saintignon, has been carried out in its most improved form by M.
Boulier. Here the pyrometer itself consists of a set of tubes one inside
the other, and all inclosed for safety in a large tube of fireclay. The
central tube or pipe brings in the water from a tank above, where it is
maintained at a constant level. The water descends to the bottom of the
instrument, and opens into the end of another small tube called the
explorer (_explorateur_). This tube projects from the fireclay casing into
the medium to be examined, and can be pushed in or out as required. After
circulating through this tube the water rises again in the annular space
between the central pipe and the second pipe. The similar space between
the second pipe and the third pipe is always filled by another and much
larger current of water, which keeps the interior cool. The result is that
no loss of heat is possible in the instrument, and the water in the
central tube merely takes up just so much heat as is conducted into it
through the metal of the explorer. This heat it brings back through a
short India-rubber pipe to a casing containing a thermometer. This
thermometer is immersed in the returning curr