HEAT OR CALORIC. 53 



(j.) " The sheet of ice which often covers the small seas, as well 

 as the rivers and lakes, not only preserves a vast body of heat in the 

 subjacent water, but when it thaws, the fish are not destroyed by the 

 cold ; for not a particle of the cold surface water can descend until 

 a change in the atmosphere has taken place, so as to raise the tem- 

 perature of the whole of the water, at least ten degrees.* 



13. Popular uses of expansion and contraction. 



(a.) Iron hoops and tires are heated red hot, and suddenly cooled 

 to bind the parts of carriage wheels, of burr millstones, &tc. 



(6.) Clocks and watches gain in cold weather, owing to the con- 

 traction of the metal, and vice versa. 



A pendulum vibrating seconds, by a change of temperature of 30 

 will alter its length about j^Vo part, which will change its rate of 

 going eight seconds a day. Or if the ball of a pendulum vibrating 

 seconds be lowered T {^ of an inch, the clock will loose ten seconds 

 in twenty four hours. Hen. 



(c.) The Compensation pendulum is easily explained, by a model 

 or diagram ; one kind, called the gridiron pendulum, consists of bars 

 of different expansibility, and having different points of support, the 

 opposite expansions balancing each other. Harrison employed 

 three bars of steel, and two of a compound of zinc and silver, and 

 they were so arranged that the expansion of the steel counteracted that 

 of the other metals, so that the pendulum did not alter in length. 

 Graham substituted for the bob of the pendulum, a glass cylinder 

 about six inches deep, and holding ten or twelve pounds of mercury, 

 the expansion of which upward, compensated for that of the steel 

 pendulum rod downward. L. u. K. 



(d.) The cracking of thick glass, by sudden heating or cooling, 

 is owing to unequal expansion ; thin glass does not crack, because the 

 heat makes its way through the glass so rapidly, that the internal and 

 external expansion are nearly alike ; otherwise there would be a 

 strain, and glass always cracks on the colder surface, whether hot 

 glass is suddenly exposed to cold, or the reverse. 



(e.) Expansion and contraction, by temperature, is capable of over- 

 coming great force. 



The two side walls of a gallery at the Conservatoire des Arts et 

 Metiers, being pressed outward by the incumbent weight, M. Molard 

 perforated the walls on opposite sides, and introduced strong iron 

 bars, whose ends were left to project beyond the walls, and were 

 furnished with strong circular iron plates, fitted on so as to screw. 



The bars, being then heated, increased in length, and the plates 

 now separated from the wall, were screwed up so as to touch it. 

 The bars, on cooling, contracted, and drew the walls closer together. 



* Parkes' Chemical Essays, VoM. p. 61. 



