made at Fort Franklin in 1825, 1826, and 1827. 245 



many of the clearest days were calm, particularly in the win- 

 ter and early spring months. In this column the maxima in- 

 crease in amount from December to March or April, and de- 

 crease gradually in the succeeding months. The irregularity 

 for July 1826 may be accounted for by only twenty-one days 

 being included ; and the smallness of the number of observa- 

 tions in the autumn of 1825 may also be considered as the 

 reason of the small mean for October. Table VII., and what 

 is nearly the same thing, the much greater sharpness and al- 

 titude of the mean curves of temperature for March and April, 

 as shewn in Plate I., f. 5, Geograph. Journal, vol. ix., may 

 also be adduced in corroboration of my remark. 



I shall not repeat here the attempt I have made elsewhere 

 to explain the cause of the clearness of the atmosphere in the 

 spring of those climates, but shall merely remind the reader, 

 that at Fort Franklin the snow does not disappear till the be- 

 ginning of May, consequently the soil cannot before that 

 month accumulate heat from day to day ; and that when the 

 snow is at the melting point, a powerful sun one day will 

 have little effect in raising the mean heat of the following 

 day. 



When the solar rays are projected at low altitudes into the 

 lower dense or cloudy stratum of the air, considerable irregu- 

 larities and sudden changes of temperature must result, pro- 

 ducing partial currents, and mingling of masses of air in dif- 

 ferent conditions, all increasing the scattering action of the 

 strata on the solar light.* But if, from the natural effect of 

 the climate, the air in the high northern regions of America 

 'be peculiarly free from clouds, and clear in the spring, there 

 does not appear to me to be any great difficulty in explaining 

 why the more oblique rays in spring should have a superior 

 effect on the black-bulb thermometer, than the more direct 

 ones passing through a comparatively cloudy atmosphere near 

 the summer solstice. I have no doubt but that future expe- 

 riments will shew that the sun, at equal altitudes, acts more 

 intensely (in spring at least) near the poles, than near the equa- 

 tor, although the increase of the temperature of the atmo- 

 sphere may be greater in the latter locality through indirect 

 radiation. 



• Vide Astronomy, by Sir J. P. W. Ilcrschel, &c, p. 33. 



