1826.] Mr. Daniell on Climate. 1 17 



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 effica- 

 cious 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 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 NEand SE inclusive is to that of the air from the other 

 quarter of the compass, in the proportion of 814 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 I have even 

 seen the difference amount 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. 



And here theory would suggest another precaution, which I 

 believe has never yet been adopted, but which would be well 

 worthy of a trial. 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 cUmate, in not being accompanied by an increase of 

 moisture. In the extremely dry state of the atmosphere to 

 which I am now alluding, the enormous exhalation from the 

 blossoms of tender fruit trees, which must thus be induced, 

 cannot fail of being extremely detrimental ; the effect of shad- 

 ing the plants from the direct rays of the sun should therefore 

 be ascertained. The state of the weather to which I refer 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 effect which 1 have pointed 

 out. 



During the time in which I kept a register of the weather, 1 

 have 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 



