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.

[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.

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 non-volatile petroleum, known as thin machinery oil.  The electromotive force obtained varies with the temperature; a pile of 295 pairs having a resistance of 95.6 ohms at 16 deg.  Cent. gave with a difference of temperature