EXPANSION BY HEAT. 



263 



among the particles. These rising and descending cur- 

 rents are shown by the arrows in fig. 287. This result 

 may be easily shown by filling a flask with Avater into 

 which a quantity of sawdust from some green hard wood 

 has been thrown, which is about as heavy as water. It 

 will traverse the vessel in a manner precisely as shown 

 in the figure. 



These results indicate the importance of applying heat 

 directly to the bottom of all vessels in which water is in- 

 tended to be heated. A considerable loss of heat often 

 occurs when the flame is made to strike against the sides 

 only of badly arranged boilers. 



EXPANSION BY HEAT. 



An important efiect of heat is the expansion of bodies. 

 Among many ways to show it, an iron rod may be so fit- 

 ted that it will just enter a hole made for the 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 



^ Fi<r. 2SS. 



W^m 



into the hole. The expansion may be more visibly shown 

 and accurately measured by means of an instrument called 

 the Pyrometer (fig. 288). The rod a h^ secured to its 



