386 Solar Radiation 



sun's rays which strike the sea have a thickness of at least 

 100 metres to work on. When they strike the land, the 

 direct effect is superficial, but the absorptive power of a 

 surface of soil is very much greater than that of a surface 

 of water, and it frequently attains a very high temperature. 

 Even in the driest countries the soil is moist, and it may be 

 that, ultimately, the surface of every particle of the soil is 

 a water surface. Whether this be so or not, when a land 

 surface cools, the heat of low refrangibility which it radiates 

 proceeds to a very large extent from water, and it is ac- 

 cordingly abundantly absorbed by the water vapour in the 

 lower layers in the atmosphere. In the absence of mechanical 

 mixture by wind, these layers can lose it only by passing 

 it on by radiation to higher layers which contain moisture, 

 whence it ultimately escapes into space. This accumulating 

 function of the atmosphere provides that while every portion 

 of the earth's surface receives heat intermittently it loses it 

 continuously. 



As the heat of the atmosphere is due to contact with, 

 or radiation from, the surface, it must be taken from the 

 supply that reaches the surface of the earth. Further, wind 

 and all mechanical atmospheric effects are due to differences 

 of density, and these are produced not only by the thermal 

 expansion and accompanying rise of temperature of the air, 

 but also, and without change of temperature, by the mixture 

 with it of a lighter gas. Such a gas is the vapour of water, 

 and the water which supplies it is at the level of the sea. 

 Therefore the sun's heat which arrives at the surface of the 

 earth at or near the sea-level has to maintain not only tlic 

 temperature of the surface of the globe, it has also to main- 

 tain all the mechanical manifestations of the air and the 

 ocean. This is the ground for asserting, as above, that the 

 only constant which is of interest in terrestrial physics is 

 the rate at which the vertical sun heats unit area of the 

 earth's surface at the sea-level. 



The instruments used for measuring the thermal effect of 

 the sun's rays must fulfil certain conditions. The area of the 

 sheaf or bundle of rays collected must be accurately known ; 



