64 TWENTIETH ANNUAL REPORT ON THE STATE CABINET. 



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that it is perfectly practicable to compute the amount of heat, and 

 to the average temperature for each part of the earth's surface, 

 as it would be if there were no variations caused by the other 

 influences just named. 



I have alluded to the fact that the heat of the sun, or rather 

 its heating power, depends upon its altitude. It varies exactly 

 with what is known to mathematicians as the sine of the sun's 

 altitude. The length of the day is also an element ; for the longer 

 the sun continues to shine on any object, the hotter it will become, 

 other things being equal. Rays of heat, also, like rays of light, 

 suffer some refraction as they pass through the air ; but on the 

 other hand, it has been proved by the experiments of Herschel 

 and Pouillet, that a part of the sun's rays are absorbed by the 

 atmosphere, or rather by the moisture that is contained in it, so 

 that only about seventy-five per cent, or three-fourths of all the 

 heat that the sun emits reaches the earth. This absorption of the 

 Ajun's heat will of course be greater, the less the sun's altitude, 

 and consequently will vary with the average of its altitude ; not 

 only for places in different latitudes, but also for the same place at 

 different seasons of the year, and for different hours in the day.* 



But in order to express the results thus obtained, in degrees of 

 temperature, as indicated by any known standard, it becomes 

 necessary to institute a proportion in which these results may be 

 compared with those obtained by actual experiment with such a 

 standard. Starting with the commonly received 80^^ Fahrenheit 

 as the average for the equator, although Humboldt gives it as 

 81*^.5, we have all the elements of such a calculation at our com- 

 mand, and the proportion is : 



As .958, the sine of the average altitude of the sun at the equatoi^, 

 multiplied by 12, the length of the day at the equator, and 

 this product multiplind by .6, the average of the sines of the 



* Of the reality of the fact above referred to, there can, of course, be no doubt ; but I 

 shall take the liberty to doubt the theory or explanation given of it, When heat is absorbed, 

 unless it becomes latent by the mass absorbing it passing from a solid to a fluid, or from a 

 fluid to a gaseous state, the absorbing mass shows the effect of the heat by an increase in its 

 own temperature. If, therefore, any portion of the sun's heat were really absorbed by the 

 air, or rather the moisture in it, the temperature of the atmosphere would be raised 

 thereby, and of course the influence of that heat would be felt no less than if it had passed 

 through the air and been returned to it by reflection or conduction from the earth. But 

 if there is anything in the air whereby it can absorb heat, it can by the same means reflect 

 it; so that it shall not reach the earth at all, but be thrown off into space, and thus be 

 totally and entirely lost in its influence upon the temperature of anything Avithin the reach 

 of our observation. And on this theory the amount of heat lost by reflection will depend 

 upon the angle at which it strikes the atmosphere, so that the correction above suggested 

 will answer as well on one theory as the other. 



