HEAT 



will rise, and colder air must be drawn in from 

 other regions to supply its place, just as we saw 

 that colder air is drawn into a room in which a 

 fire is making a draught up the chimney. In 

 this way currents of air are kept up in the 

 atmosphere, and this process of convection, as it 

 is called, is the origin of the winds, which we 

 shall have to study when considering the subject 

 of weather. 



The radiation of heat is a far more difficult 

 matter to study ; when examining it we should 

 have to ask ourselves what heat is, and how it 

 can travel across space to us from the sun, which 

 clearly can neither conduct or convect its heat 

 to us ; and also how it differs from light, which 

 likewise travels across space from the sun, and 

 perhaps also how it is connected with electricity, 

 about which we know very little, but which can 

 be obtained by means of heat, and from which 

 we can in turn obtain heat. These are very 

 difficult and deep matters, and are not thoroughly 

 understood even by the wisest men ; but they 

 are very interesting, and we must try to get some 

 ideas about them in another chapter. 



(39) 



