EXPANSION BY HEAT. 



239 



purpose in a piece of sheet-iron. If the rod be now- 

 heated in the fire, it expands and becomes larger, and 

 can not be thrust into the hole. The expansion may 

 be more visibly shown and accurately measured by 

 means of an instrument called the Pyrometer {Fig. 

 201). The rod a b, secured to its place by a screw at 



Fig. 201. 



if>££^ 



a, presses against the lever c, and this against the lever, 

 or index, d, both of which multiply the motion, and 

 render the expansion 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 multiphes twen- 

 ty times, then the 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 

 occur 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 



