772 EFFECTS OF EXPANSION. 



loosely than in the winter. As such wires possess a 

 comparatively great tension, the small elongation caused 

 by being exposed to a higher temperature is sufficient 

 to produce a very sensible depression of the middle of 

 the wire. 



Advantage is frequently taken in the mechanical 

 arts of the contraction of hot metals during cooling. 

 The walls of buildings, when leaning or bulging out, 

 may be brought into position by the alternate heating 

 and cooling of iron bars, which are firmly clamped 

 on the outside, and pass through the building. The 

 hoop of iron by which a carriage -wheel is surrounded 

 is first heated and then put on the wheel ; the whole 

 being thrown into water, the iron hoop contracts with 

 great force, and thus binds the spokes arid rim firmly 

 together. 



The same principle explains many familiar facts. 

 Thus, when a glass vessel is rapidly heated or cooled 

 it often breaks. This arises from the unequal ex- 

 pansion of the glass. If the temperature of a piece 

 of glass changes slowly, so that all parts 'expand or 

 contract uniformly, the mutual relation and cohesion 

 of the particles is not disturbed ; but if the change is 

 rapid, as, for instance, when hot water is suddenly 

 poured into the vessel, then the portion of the surface 

 in contact with the hot liquid expands much more 

 rapidly than the portion underneath, and the expanding 

 particles force those beneath them asunder; thus rupture 

 takes place, which spreads, in consequence of the brittle- 

 ness of the substance, until the crack extends right 

 through the thickness of the glass from one side to the 



