HEAT. 139 



contraction on cooling, bind the staves together. Ad- 

 vantage is also taken of this property, in the construction 

 of nice clocks and watches. As the pendulums of clocks 

 and the balance-wheels of watches, are made of metal 

 which expands by heat, and contracts by cold, the length 

 of them is altered by heat, and this of course alters the 

 rate of going: if a pendulum rod which vibrates seconds 

 be lengthened one hundredth part of an inch, the clock 

 will lose ten seconds in twentyfour hours, and if it be 

 shortened, it will cause the clock to go proportionably 

 faster. Several ingenious contrivances have been found 

 out for remedying these defects. The most common of 

 which is, what is called the gridiron pendulum, which is 

 formed of five bars, three of steel and two of a compound 

 of zinc and silver ; these are so arranged that the expan- 

 sion of the steel is counteracted by the expansion of the 

 zinc and silver, by which the pendulum is always of the 

 same length. The balance-wheels of watches are con- 

 structed upon the same principles. 



The great force with which metals expand when heat- 

 ed, was applied some years since in a singular and novel 

 manner at Paris. The two side walls of a building, which 

 were of stone, having been pressed out by the weight of 

 the floors and roof, several holes were made in the walls 

 opposite to each other ; through these, strong bars of iron 

 were placed, their ends projecting outside of the walls, 

 large plates of iron were screwed on to them. The bars 

 were then heated, which of course lengthened them, by 

 which the large plates could be advanced, and when 

 the bars cooled, the walls were drawn together. The 

 same process being repeated several times, the walls were 

 drawn to their original position. 



Liquids are expanded more by heat than solids, and 

 airs more than liquids. The expansion of liquids by 

 heat, is easily shown by experiment. Place a quantity of 

 water in a common Florence oil flask, sufficient to fill it 

 to the neck ; mark the point at which it stands when 

 cold, then apply heat; in a short time it will be seen that 

 the water is above the mark. The same will apply to 

 almost all liquids. 



Those liquids expand most by the addition of a cer- 

 tain quantity of heat, which boil with the least heat. 

 Thus mercury (quicksilver,) expands less than water, by 



