ON NORTH AMERICAN ZOOLOGY. 131 



irritability which may be supposed to take place in trees and 

 other plants during their long and complete hybernation, an 

 increased eifect is given to the sun's radiation by the clearness 

 of the atmosphere, brought about in the following manner. 

 During the intense winter colds, which are very seldom inter- 

 rupted by a rise of the temperature to the thawing point, the 

 solvent power of the air is so much diminished that almost all 

 the moisture is deposited in the form of snow*; but in spring 

 it is increased by the heating action of a sun that never sets, 

 while the ice-covered lakes, bearing so large a proportion to 

 the land in America, supply it with moisture slowly. The 

 consequence is unusual clearness of the atmosphere, enabling 

 the rays of the sun to produce their full effect. On the con- 

 fines of the arctic circle an agreeable warmth is often per- 

 ceived in the sunsliine during the months of April and May, 

 when the temperature of the air in the shade is below zero. 

 But for this adaptation of the constitution of the atmosphere to 

 circumstances, the short summer of arctic America would be 

 insufficient to clear the earth of the accumulated snovvs of nine 

 months' winter. Professor Leslie, overlooking the powerful effect 

 of direct radiation from the sun, and which indeed he could not 

 know from experimenting only in an insular climate, was led 

 from theory to fix the mean temperature of the pole at 32° F., 

 and to declare that some great error must have pervaded all 

 the thermo metrical observations of Sir Edward Parry and the 

 other arctic voyagers which showed the mean annual heat of 

 places lying near the 70th parallel to be below zero. He has 

 also described the whole of arctic America as involved in almost 

 perpetual fogs ; but this is true only of the sea-coast, and even 

 there merely in some of the summer months, when fogs are 

 produced by the intermingling currents of air of different tem- 

 peratures, coming from the heated lands, the cooler open water 

 or chillit)g masses of ice in the offing. Most of the winter nights 

 are beautifully clear, and but for the intense cold, astronomical 

 observations could be made nowhere in the northern hemi- 

 spliere with more frequency. 



* When air thus deprived of moisture is heated by admission into a warm 

 room it causes all the wood-work to shrink in an extraordinary manner, and so 

 dries and chops the cuticle of the human body that it readily becomes electric 

 by friction with the palm of the hand, till the liairs stand erect, and a peculiar 

 odour is evolved, like that which may be perceived when the rubber of an 

 electrifying machine presses hard upon the cylinder. 



