Effects Of Expansion
( Originally Published 1851 )
A cannon ball, when heated, cannot be made to enter an opening, through which, when cold, it passes readily. A glass stopper sticking fast in the neck of a bottle, may be released by surrounding the neck with a cloth taken out of warm water, or by immersing the bottle in the water up to the neck: the binding ring is thus heated and expanded sooner than the stopper, and so becomes slack or loose upon it. Pipes for conveying hot water, steam, hot air, &c., if of considerable length, must have joinings that allow a degree of shortening and lengthening, otherwise a change of temperature may destroy them. An incompetent person undertook to warm a large manufactory, by steam, from one boiler. Ile laid a rigid main pipe along a passage, and opened lateral branches through holes into the several apartments, but on his first admitting the steam, the expansion of the main pipe tore it away from all its branches. In an iron railing, a gate which, during a cold day may be loose and easily shut or opened, in a «arm day may stick, owing to there being greater expansion of it, and of the neighboring railing, than of the earth on which they are placed. Thus also the centre of the arch of an iron bridge is higher in warm than in cold weather: while, on the contrary, in a suspension or chain bridge the centre is lowered. The iron pillars now so much used to support the front walls of houses, of which the ground stories serve as shops with spacious windows, in warm weather really lift up the wall which rests upon them, and in cold weather allow it again to sink, or subside, in a degree considerably greater than if the wall were brick from top to bottom. The pitch of a piano-forte is lowered in a warm day, or in a warm room, owing to the expansion of the strings being greater than the wooden frame-work; and in cold the reverse will happen. A harp, or piano, which is well tuned in a morning drawing-room, cannot be perfectly in tune when the crowded evening party has heated the room. Bell-wires too, slack in summer, may be of the proper length in winter. There exists a most extraordinary exception, already mentioned, to the law of expansion by heat and contraction by cold, producing unspeakable benefits in nature, namely, in the case of water. Water contracts according to the law only down to the temperature of forty degrees, while, from that to thirty-two degrees, which is its freezing point, it again dilates. A very curious consequence of this peculiarity is exhibited in the wells of the glaciers of Switzerland and elsewhere, namely, that when once a pool, or shallow well, on the ice commences, it goes on quickly deepening itself until it penetrates to the earth beneath. Supposing the surface of the water originally to have nearly the temperature of the melting ice, or thirty-two degrees, but to be afterwards heated by the air and sun, instead of the water being thereby dilated or specifically higher, and detained at the surface, it becomes heavier the more nearly it is heated to forty degrees, and there-fore sinks down to the bottom of the pit or well; but there, by dissolving some of' the ice, and being consequently cooled, it is again rendered lighter, and rises to be heated as before, again to descend; and this circulation and digging cannot cease until the water has bored its way quite through.—Arnott.