1938. Kewley's alarum-thermometer (fig. 528.) consists of a glass tube (a a), about ten inches in length, hermetically sealed at one end, and united at the other to a capillary tube (b b), with an intervening and also a terminating ball (c and d). Imagine this double tube placed in a horizontal position, the largest tube, and half the intervening ball, filled with spirits of wine; and the smaller tube and half of both of the balls, with mercury. If the tube is now fixed by its centre in a brass frame (e), and nicely balanced, it is evident that every change in the temperature of the atmosphere will produce a change in the position of the centre of gravity of the tubes. One degree of heat, by expanding the spirit, will press on the mercury in the intervening ball (c), and drive part of it over to the terminating tube (d), which end will, in consequence, descend like the beam of a pair of scales or of a steam engine. Hence a moving power of great nicety and certainty is obtained, the details for the application of which, to the ringing of a bell at any distance, communicating by a wire (f), need not to be here entered into. Suffice it to say, that, by means of a scale (g), it may be set to any required temperature, and will give the alarm at a difference of even the fourth of a degree either of depression or elevation. It may be occasionally used, in gardening, to convey some idea of the changes taking place in the temperature of particular hothouses, to the head-gardener's room, in the night-time; but its most important uses are in domestic economy, hospitals, &c. This balance-thermometer, as it may be called, has been also applied, by its ingenious inventor, to the opening and shutting of windows or sashes, valves of chimneys, or flues, and steam-cocks, and either to all of these purposes at once, or to any of them.
