STUDIES IN NATURE 



bodies, as it did through the iron poker ; it is 

 convected or carried to us by the movements of 

 air or water ; it is radiated from hot objects like 

 the sun or a fire, and sent to us in invisible rays, 

 some of which are turned into heat when they 

 strike upon us. 



When, as in our former experiment, we put 

 a poker in the fire and let one end of it become 

 red hot, the other end of it may be cold ; but 

 there will be some spot along the poker where 

 it gets too hot to hold, even though that part is 

 not in the fire itself. The heat is conducted 

 along the poker from the fire. Some of the heat 

 passes to the air around, some radiates away to 

 all parts of the room, some travels along the 

 poker. If we were to put two pokers, made of 

 different substances, let us say iron and brass, 

 in the fire close together, so that their ends got 

 equally hot, and then waited for some time for 

 the heat to travel, we should find ourselves able 

 to hold them at different distances from the fire. 

 One would have a better conducting power than 

 the other, that is to say, it would let more heat 

 pass along it 



We have already said that air expands when 

 heated, and can show that this is true by our 

 experiment with an india-rubber air-ball. Now 

 when the free air which surrounds the earth 

 touches parts of the ground which have been 

 heated by the sun, some of it becomes warmer 

 than the rest. The heated parts being lighter 



(38) 



