Alarm, an instrument to give notice by sound. In its most ordinary form it consists of a bell and a hammer, combined with an escapement that lets it free at the proper time, when a descending weight or a spring makes it strike the bell. - Burglar alarms are of various forms. Some consist in an arrangement for firing a pistol, and are connected either with the lock or with the door. Some of them are so arranged as to shoot the thief at the same time that they wake up the inmates. An alarm for this purpose may always be put up at a moment's notice, by stretching a string across the hall, one end attached to the knob of a door and the other to the trigger of a pistol, or to some glass or brass vessel placed on the edge of a table or at the top of a flight of stairs, which will tumble down with a noise the moment the string is pulled by any one opening the door or crossing the hall. An alarm is easily made by arranging the wires in the circuit of a galvanic battery in such manner that the circuit may be broken when a door or window is opened; the falling of an electro-magnet which was supported by the electrical current'then gives the motive power for ringing a bell or other sound-producing instrument. - An alarm clock is a clock for sleeping rooms, provided with an alarm that may be wound up to strike at any appointed time, and so awake the sleeper.

It consists of an ordinary clock with an alarm attached, which requires to be wound up at a separate keyhole from that which winds the clock, and after each alarm requires rewinding to give it impulse for another. The alarm is commonly set, to go off at the required hour, by means of a disk which lies under the hour hand of the clock, revolving upon the same axis and with that hand. The disk has the 12 hours printed in the same order and position as on the clock face, and when this disk is brought into the same position as the clock face, that is, having the 12 on the disk at its highest point, the clock then by mechanism sets off the alarm. In order to cause the alarm to sound at 4 o'clock, for instance, the number 4 on the disk is brought under the hour hand, which latter carries the disk forward till 4 o'clock, and at this moment the 12 on the disk will be at its highest point and the alarm is set off. - The tire-damp alarm is an important invention, due to M. Chuart from France, and liberally given by him to the public. It consists of a small ball of glass or of brass suspended at the end of a lever, and containing a chemical solution highly sensitive to the gas constituting fire damp.

Long before the atmosphere has become sufficiently vitiated to be dangerous to life, or to be capable of exploding, the chemical action in the ball has altered its weight, and thus caused the lever to move and let go an escapement which sounds an alarm. - An alarm whistle is a steam whistle set on a boiler to give notice when the water falls below its proper level. For this purpose the whistle-cock is connected by a lever with a float, and opens when this float goes below a certain level. The steam rushing through the whistle sounds the alarm.