Читать книгу A Treatise on Mechanics онлайн

29 страница из 91

(28.) Let AB, fig.2. be a glass tube, with a bulb at the end A; and let the bulb A, and a part of the tube, be filled with any liquid, coloured so as to be visible. Let C be the level of the liquid in the tube. If the bulb be now exposed to heat, by being plunged in hot water, the level of the liquid C will rapidly rise towards B. This effect is produced by the dilatation of the liquid in the bulb, which filling a greater space, a part of it is forced into the tube. This experiment may easily be made with a common glass tube and a little port wine.

Thermometers are constructed on this principle, the rise of the liquid in the tube being used as an indication of the degree of heat which causes it. A particular account of these useful instruments will be found in our treatise on Heat.

(29.) The change of dimension of solids produced by changes of temperature being much less than that of bodies in the liquid or aeriform state, is not so easily observable. A remarkable instance occurs in the process of shoeing the wheels of carriages. The rim of iron with which the wheel is to be bound, is made in the first instance of a diameter somewhat less than that of the wheel; but being raised by the application of fire to a very high temperature, its volume receives such an increase, that it will be sufficient to embrace and surround the wheel. When placed upon the wheel it is cooled, and suddenly contracting its dimensions, binds the parts of the wheel firmly together, and becomes securely seated in its place upon the fellies.