LOCAL CLIMATOLOGY. 69 



of heat, arising from the very continuance of the sun's direct raj-s, 

 far beyond what is indicated. Even in the coldest days of our 

 winter, the sun's rays continue to warm whatever they fall npon, 

 as long as they fall upon it; and every object known to science 

 would not only be heated, but reduced to a fluid, or even a gas, by 

 the prolonged continuance of those rays, provided only the sub- 

 stance could be so isolated as not to part with any of its heat by 

 radiation to other objects. The al)ove results, however, corre- 

 spond, especially in the column denoting the average for the year, 

 with \ery great accuracy with the results of o])scrvation, denoted 

 by the isothermal lines, so far as those results have been ascer- 

 tained ; and I presume that the results given in columns seven 



is warmest, that the intensity of radiation multiplied into the term must 



produce a number whose ratio to the amount of heat received is constant, 



or very nearly so. 



The length of the day I have obtained by Robinson's formula, namely : 



tan. of lat. X tan. sun's decl. • ^ ,, i- . . i • i o^i 



_ = sine 01 the distance at which the sun 



radius 



reaches the horizon, from 90 deg. longitude from the place of observation. 



Converting this sine into time on the usual formulae, namely, 1 degree 

 long. =4 minutes of time, &c., and adding or subtracting, as the case may 

 require, twice the amount of time thus obtained, to 12h., we have the 

 length of the day, without correction for refraction ; and this correction, 

 as also that for the difference in the apparent semi-diameter of the sun's 

 disk, I have omitted in the foregoing computations as being too small to 

 be of importance for our present purpose. 



A much shorter method of obtaining the correction for absorption, and 

 one that is near enough for most purposes, is as follows : Divide the mid- 

 day altitude into a given number of parts, say ten : then the sum of the 

 first part will be one ordinate ; double that sum will be another; treble it, 

 a third, and so on : take the sine of the successive ordinates, and to this 

 sum add the sine of the midday altitude, and divide the sum of the whole 

 by their number increased by one, and the quotient will be the correction 

 required proximately. Thus, if the altitude be 40 deg., we have ten parts ; 

 the successive ordinates will be 0, 4, 8, 12, 16, &c. 



The results obtained by the above method, and given in the Table, differ 

 from those previously given by others (and by myself in fact), in that 1 

 have now for the first time, so far as I know, made the correction for a)>- 

 sorption. The temperature indicated, especially for the high latitudes, is 

 in consequence lower than that obtained by previous computations, and, I 

 think, more nearly corresponding with observed facts. Moreover, if the 

 effort has before been made to compute the cold of the Polar regions, it has 

 not fallen under my notice. 



