Solar Radiation 385 



When we have ascertained the supply of radiant heat 

 which reaches the earth's surface, we have to inquire what 

 becomes of it If the heat were to accumulate the world 

 would become uninhabitable. It cannot be doubted that 

 long ago the earth, in this respect, arrived at a condition of 

 equilibrium which is maintained with very slight oscillations. 

 The fundamental principle of this state of equilibrium is that 

 the heat which the whole earth receives from the sun in the 

 course of a year also leaves it in the course of a year, so tfiat, 

 taking one year with another \ the sum of the heat remains the 

 same. 



When we study the details of the annual dissipation of 

 heat we find that the atmosphere, and especially the aqueous 

 vapour in it, performs a very important part. Although prac- 

 tically transparent to the heat-rays passing from the sun to 

 the earth, it is very opaque to those leaving the earth to pass 

 outwards. They are powerfully absorbed and the tempera- 

 ture of the atmosphere is thus raised considerably above that 

 which it would have if it were as transparent to the leaving 

 rays as it is to the entering ones. This has no effect in 

 permanently detaining any of the year's supply, it still dis- 

 appears in the year, but not before it has produced important 

 climatic effects. 



We see in this differential behaviour of the atmosphere 

 towards the incoming and the outgoing rays an example of 

 Kirchoff's law, in virtue of which a body absorbs by prefer- 

 ence the rays which it itself emits. It is exceedingly unlikely 

 that any portion of the rays coming directly from the sun 

 proceed from highly heated water or water vapour ; we should 

 therefore not expect the water vapour in the atmosphere to 

 absorb them to any appreciable extent. When, however, 

 they strike the surface of the earth, whether it be land or 

 sea, they are abundantly absorbed. The blue water of the 

 ocean transmits the sun's visible rays to a considerable depth. 

 In experiments made by the writer on board the "Challenger," 

 a white surface, about four inches square, was clearly visible 

 at a depth of 25 fathoms. The total length of the path of 

 the incident and reflected ray was 50 fathoms ; therefore the 



