OF ATMOSPHERICAL MOISTURE. 131 



ness, with a south wind,' was, however, very short, 

 not exceeding one or at most two days, and was in- 

 variably accompanied with great heat and followed 

 by heavy rain ; while the north-easters last for weeks, 

 without rain and with a comparatively low tempera- 

 ture. The following statement by Mr. Thompson 

 puts this in a clear light. There occurred between 

 1826 and 1834 inclusive,— 



dryness. 



These facts sufficiently explain the fatal effects of 

 certain winds upon vegetation, the small comparative 

 value in this country of walls with north and east 

 aspects, and the general want of success that attends 

 late spring planting. Here, also, we in part discover 

 an explanation of the utility of shades interposed be- 

 tween the sun and plants newly committed to the 

 earth ; they not only cut off the solar rays, but also 

 intercept currents of air, and thus diminish the 

 amount of perspiration by two opposite methods. 



The following table, for which I am again indebted 

 to Mr. Thompson, will be found to show that the 

 average degree of dryness, in the middle of the day, 

 throughout the year, is, with a 



