186 WIND CAUSES EVAPOEATION, 



We possess, to a small extent, the power of modifymg the 

 moisture of the air, even in the open air, and have almost 

 complete control over that of glazed houses. 



It is found by experience that the effect of wind is to increase 

 the dryness of the air, and, consequently, the perspiration of 

 vegetable surfaces. It is through wind that the moisture of plants 

 and the earth is constantly borne away, and thus the evaporation 

 of plants is increased. "Evaporation," says Daniell, "increases 

 in a prodigiously rapid ratio with the velocity of the wind ; and 

 anything which retards the motion of the latter is very ef&ca? 

 cious in diminishing the amount 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 150 in 

 a high wind." Hence, the great importance, in gardens, of 

 walls and screens, which break the wind, and keep the air in 

 repose in their vicinity. The difference between the effect of a 

 given amount of cold upon the blossoms of exposed fruit trees, 

 and those of the same species trained upon walls, is well known; 

 and appears to be owing to this circumstance, much more than 

 to any difference of temperature in the two situations. 



This has heen illustrated by Howard, in the results of some interest- 

 ing experiments made by him on the annual amount of evaporation. 

 During three years, in which the evaporating gauge was placed forty- 

 three feet from the ground, the annual average result was 37 -85 inches ; 

 during other three years, when the instrument was lower and less 

 exposed, the average was 33'37 inches ; and when the gauge was upon 

 or near the ground, the annual average was only 20-28 inches, or little 

 more than half the amount evaporated in a free and elevated exposure. 



It is to be remarked that the easterly winds are, in this 

 country, both the coldest and the driest. Daniell tells us that 

 the " moisture of the air flowing from any point between N.E. 

 and S.B. inclusive, is, to that of the air from the other quarters 

 of the compass, in the proportion of "814 to '907, upon an 

 average of the whole year ; " and Mr. Thompson has found the 

 hygrometer to indicate not uncommonly from 20° to 30° of 

 dryness, during the long prevalence of the north-easterly winds 

 in spring. At the same time, the air is very cold, the effect of 

 which is to cause the sap-vessels of the stem to contract, and 



