The object of this apparatus is to close an electric circuit when the temperature of a room rises above a certain point. Many devices have been invented for effecting this object, each of which have their own advantages or disadvantages. The invention of Mr. Pritchett enables the required result to be obtained in a very satisfactory manner. The apparatus consists (as shown by the figure) of a long glass vessel containing air; connected to this vessel there is a glass tube filled with mercury. The whole is mounted on a metal cradle, which turns on pivots. According to the position which the glass vessel and its adjuncts occupy in the cradle (this position being adjustable by means of a thumb-screw, seen at the upper part of the cradle), so will the same have a tendency to rock longitudinally over to one side or the other. Now, if we suppose the position to be such that the right hand end of the glass vessel is depressed, and the left hand end raised, then if the vessel becomes 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.

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.