LIGHT AND VEGETATION 31 



towards the sky and towards the ground. These complex 

 exchanges of radiant energy are made by radiations whose 

 spectrum is different from the solar spectrum; they are always 

 electromagnetic and are situated in the infra-red, but their 

 wave-lengths He between 4 and 50 fi, the greatest intensity 

 being in the neighbourhood of 10 fi. They are often called 

 thermal radiations; actually a large part of the exchanges of 

 heat between non-incandescent bodies is made by their 

 intervention. But all radiations transport energy and are 

 also, in that sense, thermal. 



These exchanges can be summarized in the following way : 



(1) The sun heats the ground directly through the 

 atmosphere. In fact, of the 720 calories per day per sq. cm. — 

 the average quantity that we receive — one half is sent back 

 into space by reflection and does not affect us (unless it is to 

 make our globe visible to the eyes of astronomers, if any, on 

 other planets) while, of the half effectively used, 80 per cent 

 is absorbed by the ground. The atmosphere, which is rather 

 transparent to sunhght, absorbs the rest. 



(2) The ground, heated by the sun, warms, in its turn, the 

 atmosphere to which it yields 700 calories per day per sq. cm., 

 more than double that which it receives from the sun. It does 

 so, either in the form of water vapour, or, more particularly, 

 in the form of long-wave radiations (5 to 50 ix) which are 

 absorbed by the atmosphere to a large extent. Of the 700 

 calories lost by the ground, the atmosphere retains 560, letting 

 80 escape into space and reflecting 60 towards the ground. 



(3) Because it is heated up by terrestrial radiation, the 

 atmosphere, in its turn, radiates 340 calories towards the 

 ground, sending it more radiant energy than the sun. 



All these exchanges of energy are shown synoptically in 

 Fig. 8. It can be seen that they balance one another; the 

 ground, hke the atmosphere, loses as much energy as it 

 receives. 



The numbers indicated are mean values. We see from the 

 diagram in Fig. I, 8 that 360 ( 72 per cent) of the 500 calories 

 of terrestrial radiation proper are absorbed by the atmosphere. 



