Alarm, or ALARUM, is a term applied to a variety of instruments constructed for the purpose of producing sufficient sound or noise to awaken a person from sleep, or otherwise to give notice of some occurrence, or warning, of the state of the time, etc.

Fig. 1.

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Fig. 2.

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Colbert's Fire Alarm. This consists of a column of mercury in a tube, with a floating piston, which ascends and descends as the mercury expands and contracts. A rod from the piston is connected at its upper end to a lever, which, on being raised, releases a click or retent, and discharges the alarm. The apparatus is provided with a dial plate and index, pointing to the degrees of heat which is to be adjusted to a few degrees of heat above the temperature of the atmosphere, or above the utmost height to which it is expected the mercury might rise from natural causes during the night. It is enclosed in a case of open fret-work, for the purpose of readily transmitting the heat, and is to be deposited in the well of a stair-case, or other desirable place.

Congreve's Fire Alarm. The late Sir William Congreve suggested the employment of two metal plates placed in contact, with a cement between them, that would melt at a low temperature. These plates were to be suspended by a thread to opposite corners of a room, when it was considered that a slight increase of heat would melt the cement, cause the plates to fall asunder, and discharge the alarm. If the reader will refer to the experiments detailed under the article Adhesion, he will find abundant reason to doubt the certainty of the ready separation of the plates under the circumstances mentioned; and he will then probably give the preference to the following suggestion of our own. Provide a common house-bell and spring. To that end of the spring by which it is fixed to any object, tie a short piece of tape, sufficient to reach, when extended, only half way to the bell; and to that end of the spring next to the bell tie another piece of tape of the same length as the former. Then compress the spring, so that the tapes can overlap each other, and insert between them a piece of wax, (made of equal parts of common resin and bees'-wax,) which may be compressed together by the fingers.

The overlapping may be to such an extent as will cause the wax to soften and the tapes to separate, on applying a heated atmosphere to them of about 100° Fahr., when the elasticity of the spring will produce the required clattering of the bell.

We shall now proceed to give a few examples of alarums for giving notice of the arrival of predetermined periods of time, by means of easily constructed mechanism, referring the reader to Clocks and Watches for those of a more elaborate nature. The above figure represents a watch alarum, which the inventor states he has made several of, and that they answer extremely well. In a solid frame of wood p q about 8 inches by 4, and 1 thick, is inserted a metallic rod, bent into a right angle at b, to which are attached a small rod k, and two fixed pulleys de. f is a cylindrical piece of wood, having inserted at one end the pipe of a watch key, by which it may be made to rest on the pivot of the watch, (the watch being sunk a little into the frame,) and turns with the minute hand, and at the other, a pin which keeps it steady by passing through a hole in the rod k. A thread which is fixed to the piece f, and may be rolled round it, passes under the pulley e over d, and round the moveable pulley l, to which the weight w is attached; and being brought through a hole in the rod b c is fixed there by the pin g.

This pin is used to regulate the length of the thread so that when it is completely wound off the cylinder f, the weight w may rest on the plane h, which is moveable on a pin at m.

The bell is fixed to one end of a long spring r s t, the other end of which is fastened to the board at r; at t is fixed a string, which keeps the bell in the position represented in the figure, by means of a bit of wood o inserted into two notches, one in the plane h, and the other in the horizontal part of the frame n; the friction of the bit o preventing the plane h from falling. It will be easily perceived that by winding the thread a certain number of times round f, the weight w will be raised to a height from which it will take it so many hours to descend to the plane h, and that when it does reach that plane, and press upon it, the bit o will be released from the notch, and the elasticity of the spring will make the bell ring with considerable violence.

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An improved mode of releasing the bell, described in the foregoing plan, is exhibited in the subjoined diagram, wherein the parts are drawn upon a larger scale. In this figure the bit of wood o is supposed to be tightly pulled by the string attached to the spring of the bell, its lower end being detained by a fixed piece b b, and its upper end held by a bent piece of brass, which turns upon a centre at c c, and whose other end sustains the plane h by pressing against the piece a a. The descent of the weight depressing the plane h, causes the bent piece of brass to swing loose and release the piece of wood which is connected to the bell.

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The annexed figure represents a watch alarum that is sold in the shops of London, and which we have seen perform with considerable accuracy. The expense of it is only seven shillings, a a is a turned mahogany stand; b the watch laid in a velvet cushioned cavity adapted to receive it, and placed in such a position, that the hour at which a person may wish to rise, shall be placed opposite to a fixed index c. A fine line, consisting of a single horse hair, with a loop at the end of it, is then laid into the notch of a guide piece d, and the loop is then slipped over the hour hand of the watch. At e is a light ivory lever; to this the horse hair is tied about midway of its length, with a weight f suspended to its lowest end. The bell g, fixed on its steel spring, is then brought into the position shown by the line h, the extremity of which line is provided with a brass wire hook, then passed over the extremity of the lever, and put on to the upright pin i. When, by the process of time, the hour hand has arrived at the period proposed, which is opposite to the point of the index, the horse hair slips from it, the little weight thereby becomes unsupported, pulls down the ivory lever, raising the hook, of the pin, which, releasing the spring, sets the bell ringing.

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The periodical journals a few years ago abounded with plans of simple alarums; and any person having a taste for such trifles in mechanics might easily multiply them, as the materials as well as the arrangements of parts may be almost infinitely varied. Sand, passing through a minute perforation, (as in the hour-glass,) and charging a receptacle, whose weight in due time gave motion to a bell, was a common expedient. The substances used for domestic light have also been called into use, to show by their uniform decrease of quantity the time passed, and by their decrease of weight in consequence allow the reaction of a constant force to give motion to an alarum. The most perfect and elegant piece of mechanism for this purpose, is Berrollas's patent watch alarum, which we have fully described under the head Horology.