NOCTURNAL RADIATION. 261 



years ; but in some seasons one of these months, in others 

 another, is the coldest, the temperature being ruled greatly 

 by the prevailing winds. 



Generally speaking, the mean annual temperature of 

 places in the interior of Xorth America falls within a 

 degree or two of the mean heat of the two months of 

 April and October. The mean temperature of the whole 

 surface of the earth is, according to Dove's calculations, 

 58-2° Fah., being 54° in January and 62° in July. For 

 such a mean annual heat in America we must descend to 

 the 34th parallel of latitude ; but the July heat of 62" 

 Fah. extends northwards to the Mackenzie. 



The intense winter colds in the high latitudes are appa- 

 rently in a great measure owing to the active nocturnal 

 radiation into the clear blue sky. The observatory, 

 which was a small log building without a fire-place, fur- 

 nished us with the means of judging how much greater 

 the depression of temperature in the night was in places 

 exposed to the sky, than in those covered in. 



The daily curve of atmospheric temperature for the three 

 winter months was a bold and nearly regular hyperbolic 

 curve, of which the mean was — 25'2° Fah., the maximum 

 — 22*2° Fah., and the minimum — 26*7° Fah.* The maxi- 

 mum occurs at lh. 18m. P.m., and the mean line is crossed 

 by the curve at 9h. 19m. a.m., and 6h. 28m. p.m.; the 

 lowest temperature being reached at 7 a. m. The ascend- 

 ing branch of the curve, therefore, corresponds to an in- 

 terval of 6h. 18m., and the descending one to 17h. 42m. 

 During 14h. 51m. the temperature is below the mean, and 

 it is only 9h. 9 in. above it, which indicates a tolerably 

 bold curve in the day, and a nearly horizontal course in 

 the nio;ht. 



* The correction for the error of the thermometer at low tempera- 

 tures used in Tab. X. col. 8. was not applied to these numbers. 



s 3 



