288 Transactions, of the Horticultural Society. 



The improvement of natural climate consists in avoiding 

 the extremes of drought and cold produced in nature, by 

 wind and radiation. 



" The amount of evaporation from the soil, and of exhalation 

 from the foliage of the vegetable kingdom, depends upon two 

 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, vapour 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 little or no 

 control 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. Exces- 

 sive 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 

 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 mois- 

 ture, would yield 125 in a moderate breeze, and 150 in a high 

 wind." 



The wind from N.E. to S. E., is drier than that from any 

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

 upon an average of the year. During die prevalence of 

 such weather in Spring, the warmth produced by the sun, for 

 example against a south wall, is sometimes more injurious 

 than useful, by not being accompanied with an increase of 

 moisture. The enormous exhalation from the blossoms, thus 

 induced, is extremely detrimental ; and requires shading from 

 the direct rays of the sun. This state of the weather often 

 occurs in April, May, and June, but seldom lasts many hours. 

 Great mischief, however, may arise in a very small interval of 

 time, and the disadvantage of a partial loss of light cannot 

 be put in comparison with the probable destruction of blossom 

 likely to be incurred. The shelter of mats or of bunting on 

 such occasions would often prevent the sudden injury which 

 so frequently arises at this period of the year. 



Over the absolute state of the vapour in the air we are 

 wholly powerless, and by no system of watering can we affect 

 the humidity of the free atmosphere. This is determined in 

 the upper regions ; it is only, therefore, by indirect methods, 

 and chiefly by the selection of proper screens, that we can 



