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._

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A STANDARD THERMOPILE.