240 HEAT. 



expanding, it becomes large enough 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. Hogsheads are firmly hooped with iron 

 bands in the same way, with more force than could be 

 ever given by driving on with blows of the mallet. 



This principle was very ingeniously applied in draw- 

 ing 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 cir- 

 cular plates of iron were screwed on these projecting 

 ends. The bars were then heated, which increased 

 their length ; the plates were then screwed closely 

 against the walls. On cooling, they contracted, and 

 drew the walls nearer together. The process was re- 

 peated on alternating bars, until the walls were re- 

 stored to their perpendicular positions. 



All tools, where the wooden handles enter iron sock- 

 ets, will hold more firmly if the metal is heated before 

 inserting 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 by- 

 stander suggested that the outer piece be heated, keep- 

 ing the inner cool, when a force of less than ten pounds 

 quickly separated them. In other cases, where the 

 large iron nuts have been thoughtlessly screwed, while 

 warmed with the hands, on the cold metallic axles of 

 wood-sawing machines in winter, they have contracted 

 so that the force of two or three men has been insuffi- 

 cient to turn them. 



