Chap, xxxv.] RADIATION OF HEAT. 459 



place being taken by a cold layer, which in turn 

 becomes heated, and rises, to be replaced by another 

 cold layer. This distribution of heat through the 

 liquid by convection will go on till the whole mass is 

 of one temperature. Gases also have little conducting 

 power, but can also distribute heat by convection. 

 If, in a mass of air, one stratum be heated, a current 

 is produced, the heated air giving place to colder. 

 The trade winds are examples of convection currents 

 in air. The air heated at the equator flows to both 

 sides and high in the atmosphere, while colder cur- 

 rents flow towards the equator and lower in the 

 atmosphere to take its place. 



Radiation of heat. On a winter's day we 

 may feel warmed by the rays of the sun, even although 

 the air by which we are surrounded is below the 

 freezing temperature. As soon as the smallest obstacle 

 intervenes to cut the sun's rays off from us, we feel 

 the sudden withdrawal of heat, although the sun's 

 rays may be cast all about us. We do not feel the 

 warmth, that is to say, unless the rays are directly 

 falling upon us. These facts show that the warmth 

 is not communicated to us by the air, but that the 

 rays from the sun travel through the air without 

 heating it. In the same way, if we stand before a 

 fire we feel the heat emitted by it ; and it can be 

 shown that the t.lr surrounding is not warm enough 

 to communicate the heat experienced, that the heat 

 passes from the fire without warming the air through 

 which it passes. A red-hot ball suspended in a room 

 will be felt by the hand held at some distance from 

 it, to be very warm ; but if a thermometer be screened 

 from the ball it will not indicate any high tempera- 

 ture. A lens of ice can be made to focus the rays of 

 the sun upon a substance to set it on fire, just as a 

 burning glass would do, and yet the heat is not im- 

 parted to the ice which conveys it. 



