78 ATMOSPHERIC MOISTURE, CONSIDERED 



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. 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 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. 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 flowing from any point between N.E. and S.E. inclusive, 

 is to that of the air from the other quarter of the compass in the proportion 

 of 81 4 to 907, upon an average of the whole year: and it is no uncommon 

 thing in spring for the dew-point to be more than 20 degrees below the 

 temperature of the atmosphere in the shade, and the difference has even 

 amounted 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 doubtful ; and in 

 the case of tender fruit-trees, such screens should not be too far apart. 



246. When trees are trained upon a wall with a southern aspect, they 

 have the advantage of a greatly exalted temperature ; but this temperature, 

 in spring, differs from the warmth of a more advanced period of the year, or 

 of a more southern climate, in not being accompanied by an increase of 

 moisture. In this extremely dry state of the atmosphere, the enormous 

 exhalations from the blossoms of tender fruit-trees which must thus be in- 

 duced cannot fail of being extremely detrimental ; the effect of shading 

 the plants from the direct rays of the sun should therefore be ascertained. 

 The state of the weather referred to 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 good effect of shading, by mats or 

 canvas, at the distance of a foot or two from the wall." (Idem.) 



247. Mr. Daniell " kept a register of the weather, and has seen, in the 

 month of May, the thermometer in the sun at 101, while the dew-point was 

 only 34 : the state of saturation of the air, upon a south wall, consequently, 

 only amounted to 120; a state of dryness which is certainly not surpassed by 

 an African harmattan. The shelter of a mat on such occasions would often 

 prevent the sudden injury which so frequently arises at this period of the 

 year." With great submission to Mr. Daniell, who must necessarily know 

 so much more of the subject than we can do, we cannot help thinking that 

 this statement must be somewhat exaggerated. In this country we certainly 

 have the sun frequently sufficiently powerful in summer to raise the ther- 

 mometer in the free air, at a distance from the wall, to 101, whilst the air 



