NOVEMBER. 



499 



circumstances, the saturation of the air with moisture, and 

 the velocity of its motion. They are in inverse proportion to 

 the former, and in direct proportion to the latter. 



"When the air is dry, vapor ascends in it with great rapidity 

 from every surface capable of affording it, and the energy of 

 this action is greatly promoted by wind, which removes it 

 from the exhaling body as fast as it is formed, and prevents 

 that accumulation which would otherwise arrest the process, 



"Over the state of saturation the horticulturist has no con- 

 trol in the open air, but over its velocity he has some com- 

 mand. He can break the force of the blast by artificial 

 means, such as walls, palings, hedges or other screens ; or he 

 may find natural shelter in situations upon the acclivities of 

 hills. Excessive exhalation is very injurious to many of the 

 processes of vegetation, and no small proportion of what is 

 commonly called blight may be attributed to this cause. 

 Evaporation increases in a prodigiously rapid ratio with the 

 velocity of the wind, and anything which retards the motion 

 of the latter, is very efficacious in diminishing the ascent of 

 the former ; the same surface which in a calm state of the 

 air would exhale 100 parts of moisture, would yield 125 in 

 a moderate breeze, and 1.50 in a high wind. The dryness of 

 the atmosphere in spring renders the effect most injurious to 

 the tender shoots of this season of the year, and the easterly 

 winds especially are most to be opposed in their course. The 

 moisture of the air blowing from any point between northeast 

 and soirtheast inclusive, is, to that of the air from the other 

 quarter of the compass, in the proportion of 814 to 907 upon 

 an average of the whole year ; and it is no uncommon thing 

 in the spring for the dew point to be more than 20 de- 

 grees below the temperature of the atmosphere in the shade, 

 and I have seen the difference amount to 30 degrees. The 

 effect of such a degree of dryness is parching in the extreme, 

 and if accompanied with wind, is destructive to the blossoms 

 of tender plants. The use of high walls, especially upon the 

 northern and eastern sides of a garden, in checking this evil, 

 cannot be doubted, and, in the case of tender fruit trees, such 

 screens should not be too far apart." 



