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IMPROVED PYROMETER.

Mr. John Frew, Langloan Iron Works, Coatbridge, has been successful in
perfecting a most ingenious pyrometer, an instrument which is capable of
continuously indicating every variation of temperature with a remarkable
degree of correctness. This instrument, which we here illustrate, has
already become known to a number of proprietors and managers of blast
furnaces; and on the occasion of the members of the Iron and Steel
Institute visiting Coatbridge, in connection with the meeting of that
body which was held in Glasgow last autumn, many persons became
interested in its construction and in the practical determination of
blast temperatures by its readings. Furthermore, Sir William Thomson has
expressed himself as being highly delighted with it on account of the
manner in which its use illustrates various beautiful scientific
principles.

The leading principle on which the construction of this pyrometer has
been based is the well-known law of the expansion of gases. Referring to
our engraving, it will be seen that at A is a pipe through which air from
the cold blast main is admitted into another and larger pipe, B, which
reaches nearly to the bottom of a water cistern, C. By means of the inlet
and outlet pipes, D and E, the height of the water in the cistern is
maintained at a uniform level. In this way there is provided a head of
water which retains within the pipe, B, a constant pressure of air,
equivalent to the head of water between the open end of that pipe and the
overflow at E. Any excess of pressure is prevented by means of the
open-ended pipe, which permits the air to escape by the central tube.
This latter prevents the agitation caused by the upward rushing air from
disturbing the level of the water in the cistern; and in order further to
assist this, the central tube is filled loosely in its upper part with
lead bullets or other suitable materials supported on a perforated plate.
The water level in the cistern is indicated by means of a glass gauge,
which is represented at G. To the upper end of the pipe, B, another pipe,
H, is attached. This is required for conveying the cold air to the
pyrometer proper, for the piece of apparatus above described is simply an
arrangement for securing a flow or current of air at constant pressure.

At any point where it is desired to fix a pyrometer, a connection is made
with the pipe last spoken of, by means of