THE ORIGIN OF ORGANIC MATTER 31 



It is important to bear in mind that the value for the solar constant 

 (Abbot 1.95 and Bigelow 3.98) refers to the intensity at the outside of 

 the atmosphere. While the establishment of the true value of this influx 

 of energy is of very great importance, it has only indirect bearing on the 

 problem of photosynthesis. In calculating the amount of solar energy 

 available on the earth, the value of the solar constant has occasionally 

 been used. That this is erroneous is evident. Fortunately there appears 

 to be little dispute regarding the amount of solar energy received at the 

 surface of the earth and it is this factor which is of more immediate 

 importance to the problem of photosynthesis. 



Abbot has summarized the total solar radiation on normal incidence 

 and horizontal surface assuming the sun to shine 261,000 minutes per year 

 and has calculated the square feet required per horse-power on the basis 

 of complete absorption and transformation. Table 2 is taken from Abbot's 

 "The Sun," page 386. 



TABLE 2 



Abbot ^^ makes the following statement to give an idea of the total 

 amount of energy received by the earth from the sun : "Expressed in 

 another way, the measurements indicate that if the sun's rays could be 

 completely employed to melt ice exposed continuously to them at right 

 angles, they would suffice to melt a layer 426 feet thick in a year. Such 

 a layer at the earth's mean distance, if it entirely surrounded the sun. 

 would weigh 4 X 10^^ (4 followed by 25 ciphers) tons, and the complete 

 melting of it each year would represent as many heat units as the burning 

 of 4 X 10-^ tons of anthracite coal. This, then, is a measure of the sun's 

 yearly output of radiation." 



It is a very interesting fact that the value of the solar constant is sub- 

 ject to certain periodic fluctuations. These are, firstly, long period variations 

 which indicate that the emission of radiation varies with solar activity as 

 made evident by sun-spots and other phenomena. With increasing num- 

 ber of sun-spots solar radiation increases, though the relation is not a di- 

 rect numerical one. Thus Abbot reports that the increasing solar activity 

 of 1914 resulted in a 3.5 per cent increase in the value of the solar constant 

 over 1913. The solar constant is also subject to short-period variations. 



"Abbot, The Sun, p. 299. 



