subjected to an elevation of temperature,
the air inside the same will become expanded, and the mercury column
in the tube will be driven over to the left, and will rise in the
turned up end of the tube. This will cause the left hand branch of the
glass vessel, and its attachments, to become increased in weight,
while the right hand branch will become proportionally lighter; the
consequence of this will be that the vessel and its cradle will cant
over, and by falling on an electrical contact will close a circuit and
sound an alarm. It is obvious that the apparatus is equally well
adapted for indicating a diminution as well as an increase of
temperature, for if the electrical contact be placed under the right
hand portion of the cradle, and the latter be adjusted so that in its
normal position its left hand portion is depressed, then when the
glass vessel becomes cooled, the air in it will contract, and the
mercury will fall in the turned-up portion of the tube before referred
to, and will rise in the limb connected to the vessel, consequently
the cradle and glass vessel will cant over in the reverse way to that
which it did in the first case.

Owing to the surface which the glass vessel exposes, the air inside
quickly responds to any external change of temperature, consequently
the apparatus is very sensitive. Another important feature is the fact
that the cradle and vessel in canting over acquires a certain
momentum, and thus the contact made becomes very certain.

[Illustration: PRITCHETT'S ELECTRIC FIRE ALARM.]

Mr. Pritchett proposes that his apparatus shall give external evidence
outside the house by ringing a gong, and by dropping a semaphore arm
released by an electromagnet. He also proposes (as has often been
suggested) that a water supply shall be automatically turned
on.--_Electrical Review._

* * * * *

A STANDARD THERMOPILE.

Dr. G. Gore, F.R.S., has invented an improved thermopile for
measuring small electromotive forces. It consists of about 300 pairs
of horizontal, slender, parallel wires of iron and German silver, the
former being covered with cotton. They are mounted on a wooden frame.
About 11/2 in. of the opposite ends of the wires are bent downward to a
vertical position to enable them to dip into liquids at different
temperatures contained in long narrow troughs; the liquids being
non-conductors, such as melted paraffin for the hot junctions, and the
no