PROPORTION OF EVAPORATION, 63 



explain itself quite naturally, and without calling in any special 

 action of vegetation, if the open space were less accessible to the 

 rainy wind than was the wooded clump. 



" It is worthy of remark that the mean proportion between the 

 two instruments, which was 0-939 far 1868, had been 0-938 for 1867, 

 and 0'952 for 1866 ; that is to say, that the proportion varied very 

 little." 



Along with the allowance, which it may be deemed necessary to 

 make for the quantity of rain which may be arrested by the foliage 

 and evaporated thence v^ithout reaching the ground, in a rough 

 estimate of the benefit which trees may bring to the soil by 

 preventing evaporation thence, it may be deemed necessary to make 

 some allowance in certain circumstances for the evaporation, through 

 the stomates of the leaves, of moisture which is dissipated in the 

 atmosphere, if this be in excess of the difference between what is 

 evaporated from the soil under the shade and shelter of the wood and 

 what is evaporated from adjacent open country. 



I am disposed to question whether such a case be of frequent 

 occurrence, or if it be likely to occur at all, but the possibility of its 

 occurrence must not be ignored. I cannot do more than refer to it 

 to show that it is not ignored by me. 



In a preceding section I have referred to the observations of Mr 

 Blore on evaporation, under shade and shelter, and in open ground, 

 at the Cape of Good Hope ; and in connection with a notice of these 

 observations I had occasion to cite some experiments on evaporation 

 made by M. Mathieu. A report of these was published by the 

 French Government in the " Atlas Meteorologique de V Ohservatoire 

 Imperial, 1867," from which it appears that, during the seven months 

 comprised between April and October in that year, whilst the 

 quantity of water evaporated from forest leaves was 3.23 inches, the 

 quantity evaporated from land clear of forest was 16.29 inches, or 

 about five times as much. 



In April, bafore the development of leaves, if 1,000 represented the 

 evaporation from the open country, 623 represented the evaporation 

 from woodland ; but after the trees became clothed with foliage the 

 amount of evaporation from the woodland was only 130, as against 

 1,000 of open country evaporation. In October the woodland evapora- 

 tion was to the open country evaporation as 90 to 100. 

 The experiments were continued in 1868. 



