52 HEAT. 



are made originally too small for the frames they are 

 to surround.. They are' then heated red hot and ap- 

 plied in a state of expansion. The contraction which 

 afterward takes place, on sudden cooling hy cold wa- 

 ter, binds the wooden frame-work together with the 

 greatest firmness. So in making steam-boilers, the 

 rivets are fastened while hot, that they may by subse- 

 quent contraction unite the plates more firmly. 



112. HOT-WATER PIPES. In certain 



What disad- . . . ^ 



vantages arise uses to which iron is applied, the conse- 

 ^ansion. 6 o/*~ ( l uences ^ expansion have to be carefully 

 metals? guarded against. A cast-iron pipe for the 



conveyance of steam or hot water, must not be so laid 

 that its ends touch two opposite walls, lest by its ex- 

 pansion when heated, the walls should be overturned. 



113. CLAMPS IN WALLS. If the two 



ends of a P iece of metal are fixed so that 



clamps in they cannot move, and contraction takes 



walls ? 



place by cold, the metal must break. Cast- 

 iron clamps in walls are frequently thus broken. If 

 they are of wrought iron, they often crush the stone, 

 and thus loosen themselves in their sockets. 



114. LIFTING WALLS. Walls of build- 



Mow are walls 



straightened ings in danger of falling, have been restored 

 b <Ld XP contr- to tnen * perpendicular position by taking 

 tion? indirect advantage of expansion. This 



is eifected, by connecting the walls to be lifted into 

 place, by an iron rod, fixed firmly into one wall, and 

 passing loosely through a hole in the other. The 

 whole length of the rod is then heated by lamps, 

 whereby expansion is occasioned, and the rod made to 



