OP ATMOSPHERICAL MOISTURE. 129 



"We possess, to a certain extent, the power of modi- 

 fying 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. "Evapora- 

 tion," says Mr. Daniell, " increases in a prodigiously 

 rapid ratio with the velocity of the wind ; and any- 

 thing which retards the motion of the latter is very 

 efficacious 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* 



for 14 years (1826-1839 inclusive,) as deduced from observations 

 made under the direction of the Regents of the University, and col- 

 lected in their Report presented March, 1840, is 84 - 40 inches. The 

 highest average (from 64 stations) for any single year is 44 '40 (A D. 

 1827) ; the lowest (for 1839) 32-10 inches.] 



* This has been illustrated by Mr. Howard, in the results of 

 some interesting 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 instru- 

 ment was lower and less exposed, the average was 33*37 inches; and 



6* 



