nals at Portsmouth and Rye amounted to six
miles.--_Science._

* * * * *

PYROMETERS.

The accurate measurement of very high temperatures is a matter of great
importance, especially with regard to metallurgical operations; but it is
also one of great difficulty. Until recent years the only methods
suggested were to measure the expansion of a given fluid or gas, as in the
air pyrometer; or to measure the contraction of a cone of hard, burnt
clay, as in the Wedgwood pyrometer. Neither of these systems was at all
reliable or satisfactory. Lately, however, other principles have been
introduced with considerable success, and the matter is of so much
interest, not only to the practical manufacturer but also to the
physicist, that a sketch of the chief systems now in use will probably be
acceptable. He will thus be enabled to select the instrument best suited
for the particular purpose he may have in view.

The first real improvement in this direction, as in so many others, is due
to the genius of Sir William Siemens. His first attempt was a calorimetric
pyrometer, in which a mass of copper at the temperature required to be
known is thrown into the water of a calorimeter, and the heat it has
absorbed thus determined. This method, however, is not very reliable, and
was superseded by his well-known electric pyrometer. This rests on the
principle that the electric resistance of metal conductors increases with
the temperature. In the case of platinum, the metal chosen for the
purpose, this increase up to 1,500 deg.C. is very nearly in the exact
proportion of the rise of temperature. The principle is applied in the
following manner: A cylinder of fireclay slides in a metal tube, and has
two platinum wires one one-hundredth of an inch in diameter wound round it
in separate grooves. Their ends are connected at the top to two
conductors, which pass down inside the tube and end in a fireclay plug at
the bottom. The other ends of the wires are connected with a small
platinum coil, which is kept at a constant resistance. A third conductor
starting from the top of the tube passes down through it, and comes out at
the face of the metal plug. The tube is inserted in the medium whose
temperature is to be found, and the electric resistance of the coil is
measured by a differential voltameter. From this it is easy to deduce the
temperature to which the platinum has been raised. This pyrometer is
proba