50 



HEAT. 



proportion to their density ; this is by no means uni- 

 versally the case. 



105. THE OCEAN A RESERVOIR AND 



How does the 



ocean serve as REGULATOR OF HEAT. In hot Weather the 



a reservoir -L -L >i i c -\ if 



and regulator ocean absorbs the heat of the air. If it 

 of heat? were an ocean of mercury, it would soon 

 grow as hot as the air, and therefore cease absorbing 

 but its capacity for heat is so much greater that this 

 does not occur. Again, in cold weather, it is con- 

 stantly giving out the large quantity it has absorbed, 

 but at the same time itself grows cool, though very 

 slowly. It is thus a reservoir of heat and a regulator 

 of climate. 



106. FIRE BY COMPRESSION. The fire 

 principle of svr i n g e > represented in the figure, is an 

 the Fire Sy- instrument designed to produce fire 

 by the compression of air. On forcing 

 the piston suddenly down, the tinder below it is 

 ignited. This takes place on the principle already 

 explained. The specific heat of compressed air 

 is less than that of air uncompressed. When the com- 

 pression takes place, the surplus elevates the tempera- 

 ture and inflames the tinder. 



EXPANSION. 



107. EXPANSION UNIVERSAL. All bodies, 



ha* at hea/ e on solid > liquid, and gaseous, expand by heat, 



dun"?* ^ b ~ and contract to tneir original dimensions on 



cooling. An iron wire lengthens by heat : 



the mercury in a thermometer expands and rises by 



