384 Solar Radiation 



one spot or another of the earth's surface. Therefore our 

 first step should be to choose a locality where the sun passes 

 through the zenith at mid-day. 



Before reaching the sea-level the sun's rays have to pass 

 through the whole thickness of the atmosphere. It is a 

 matter of every-day observation that the atmosphere varies 

 in transparency. The second condition is therefore to put 

 ourselves in the position of greatest advantage as regards 

 atmospheric conditions. Clouds and similar visible obstruc- 

 tions are of course excluded. The air should be motionless, 

 the sky should be clear and of a deep blue colour in the 

 regions remote from the sun and should contain nothing that 

 can be called haze, or that interferes with the definition of 

 the sun or other heavenly bodies. 



From inspection alone we can only approximately ascer- 

 tain what are the most favourable meteorological conditions. 

 For this reason it is necessary to multiply observations and 

 never to miss fine weather. In the end we cannot fail to 

 approach nearer and nearer to the exact determination of 

 the maximum heating power of the sun on the earth's 

 surface at or near the sea-level, in so far as the degree 

 of perfection of our instrumental resources permits. This 

 limitation imposes on us the duty to continue observations, 

 not only until the best natural conditions have been found, 

 but also so long as the instruments or experimental methods 

 appear to be capable of improvement. If we suppose for 

 one moment that we have arrived at the point where no 

 further improvement is possible, then the result of our work 

 is the determination of the rate at which unit area of the 

 earth's surface at or near the sea-level receives heat from the 

 vertical sun in unit time. 



There is no question here of how much is lost on the way 

 from the sun. All that is sought, and the most that is ascer- 

 tained, is how much arrives. If we multiply this by the area 

 included in the great circle of the earth we have tJte amount of 

 radiant heat which we can count on as being supplied to the 

 whole earth in unit of time. This is the constant wJiicJi is of 

 greatest importance in physical geography. 



