HEATING BY HOT "VVATf.:-^, HOT AIR, AND STEAM. 175 



of heat, and parts with it slowly to the air by which it is sur- 

 rounded. 



Water is a better conductor of heat than air. Every gardener 

 well knows how rapidly a wet mat, or any other wet substance, 

 will carry off the heat in a frosty night, if laid over a hot-bed, 

 or green-house. In fact, the temperature of a frame under such 

 covering will fall quicker than if fully exposed. Yet the case 

 is different if the mat be dry, because the apertures of the mat, 

 and also the space between it and the glass, are filled with air 

 at rest, — because the latter is a bad conductor of heat, and the 

 former a good conductor. In a tank of water in a hot-house, 

 the thermometer will indicate a temperature probably 10° above 

 the atmosphere, while, by plunging the hand in the water, it will 

 feel about 10° lower. This arises from the power possessed by 

 the water of conducting the heat from the hand immersed in it. 

 The effect in all these cases may appear different, but the prin- 

 ciple of action is the same. Water conducts heat rapidly from 

 a body warmer than itself, and conveys it to a colder one. 



Let a stream of air be forced through a tube 100 feet in 

 length, entering at the temperature of 150° ; by the time it has 

 travelled, by its own specific gravity, to the end of the tube, 

 it will be reduced to the temperature of the external atmosphere. 

 A stream of water, under the same circumstances, will travel to 

 the end of the tube with a very slight diminution of its tem- 

 perature, probably only a few degrees, and will have heated the 

 tube, if a good conductor, to nearly the same temperature as 

 itself during its passage. 



