126 EVAPORATION AND WIND. 



the air; and the perspiration of vegetables; so that anything retarding 

 the force of the wind diminishes, of course, the perspiration. A 

 surface exhaling 100 parts of moisture in a calm air yields 125 in a 

 moderate breeze and 150 in a high wind ; hence the advantage of 

 walls and screens to break the force of wind. The difference in fruit 

 trees trained upon the walls and in exposed situations is owing princi- 

 pally to this circumstance. The amount of evaporation 43 feet from 

 the ground, is annually about 37.85 inches ; the evaporation guage 

 being placed lower and less exposed the annual average is 33,37 and 

 when the guage is near, or upon the ground, it is 20.28 inches. The 

 easterly winds are said to be the coldest and dryest in England. The 

 mean temperature, dryness and moisture of the air with respect to the 

 wind, as appears from observations in the London Horticultural 

 Society is south wind; temperature 51.4; dryness 4.2, moisture 8.77. 

 South wind; temperature 52.2; dryness 4.7, moisture 8.59. "West 

 wind; temperature 51.3 ; dryness 6.2 ; moisture 7.33. North wind; 

 temperature 46.2; dryness 6.0; moisture 7.49. 



The radiant power of heat may be advantageously modified by 

 screens. By these the radiant heat of the sun is intercepted and 

 returned into space ; thus the perspiration and the temperature sur- 

 rounding a plant are diminished ; the radiant heat of the earth is also 

 intercepted and returned to the earth. Thus the temperature may be 

 modified, when necessary, by art. In northern climates garden walls 

 are often blackened by which plants near them are favored in their 

 growth by the increased heat. Cold air being heavier than warm, a 

 great difference will be observed in plants growing in a valley and on 

 more elevated situations. This however is less remarkable in the 

 wide valleys of the U. S. especially those of our rivers and lakes. 

 The valley of the Hudson, with from one half to one mile marginal 

 width, is protected from autumnal frosts ; but beyond that, plants suf- 

 fer much. The warm vapor rising during a cold night from the 

 water is thought to protect adjacent shores. Our large lakes thus 

 have an important influence on the neighboring soil. A garden placed 

 on a gentle slope" having a southeastern aspect, with a running stream 

 at its base, secures the greatest advantages of position. Plants in 

 this climate, however, where the frequent bright suns of winter tem- 

 porarily thaw the plants, a more northern aspect is preferred, espe- 

 cially if they are not covered by matting, straw, or boards during the 

 winter, which, with all delicate plants, should not be omitted. The 

 air in warm climates is generally filled with vapor to a greater extent 

 than in colder ones, though it is occasionally more dry ; hence the 

 object of glazed houses is to effect the same conditions, when provided 

 with hot water pipes. As general rules, moisture is most required 

 when plants begin to grow, and least when their growth is completed ; 

 2d, the atmospheric moisture required by plants ; all things being equal, 



