DEOPPING FROM LEAVES. 37 



there being in excess of what saturates it, in accordance with the 

 elements by which that dew-point is determined. These are ascer- 

 tained by observation of the difference of temperature indicated by a 

 wet and a dry bulbed thermometer, of the barometric pressure, and 

 of the elastic force of the vapour at the temperatures, and under 

 such pressure. The elements are numerous, but the formula of 

 calculation simple, and copious tables for facilitating this have been 

 published. 



In regard to the eflfects of different trees, I find it stated by a 

 writer on the subject, in " The Farmers' Magazine," Mr Cuthbert W. 

 Johnstone, that " the dew under some large oak trees, by the side 

 of his bowling green, at Croydon, is always considerable of an 

 evening ; but that under a witch-elm growing by Iheir side the dew ia 

 almost always absent." In Europe, generally, it has been found that 

 the most copious humidity or moisture has been produced by the 

 elm, and, in decreasing order, by the poplar and horse-chestnut, while 

 the least effect is produced by firs. 



