Colberl'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 con tracts. A rod from the piston is connected at its upper end to a lever, which, on being raised, releases a click or detest, 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 em ployment 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 ex tended, only half way to the bell; and to thin 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 com pressed 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 Ctocks AND Wsrcnes 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 g 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 d e. 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 fame,) 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 1, to which the weight w is attached; and being brought through a hole in the rod b e 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 in. The bell is fixed to one end of a long spring r a 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 ti ; the friction of the bit o pre venting 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 consi derable violence.
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.