264 



HEAT. 



place by a screw at , j^resses against the lever c, and this 

 against the lever or index d, both of which multiply the 

 motion, and render the exj)ansioii very obvious to the eye 

 when the rod is heated by the lamps. If the rod should 

 expand one-fiftieth of an inch, and each lever multiplies 

 twenty times, then tlie index (or second lever) will move 

 along the scale eight inches ; for 20 times 20 are 400, and 

 400-50ths of an inch are 8 inches. 



Many cases showing the expansion of heated bodies oc- 

 cur in ordinary practice. One is afforded by the manner 

 in which the parts of carriage wheels are bound together. 

 The tire is made a little smaller than the wooden part of 

 the wheel; it is then heated till, by expanding, it be- 

 comes large enougli to be put on, when it is suddenly 

 cooled with water, and, by its powerful contraction, binds 

 every part of the wheel together with great force. Hogs- 

 heads are firmly hooped with iron bands in the same way, 

 with more force than could ever be given by driving 

 with blows of the mallet. 



This principle was very ingeniously applied in drawing 

 together two expanding brick walls of a large building in 

 Paris, which threatened to burst and fall. Holes were 

 drilled in the opposite walls, through which strong iron 

 bars across the building projected, and circular plates of 

 iron were screwed on these projecting ends. The bars 

 were then heated, which increased their length; the i)lates 

 were next screwed closely against the walls. On cooling, 

 they contracted, and drew the walls nearer together. The 

 process was repeated on alternating bars, until the walls 

 were restored to their perpendicular positions. 



All tools, where the wooden handles enter iron sockets, 

 will hold more firmly if the metal is heated before insert- 

 ing the wood. 



The metallic parts of pumps sometimes become very 

 difficult to unscrew, and a case has occurred where two 

 strong men could not start the screws, until a bystander 



