82 EFFECTS OE FOEESTS ON HUMIDITT. 



the ground the more rapid ia this evaporation. The effect may be 

 seen in the steaming of the streets after a short shower in summer ; 

 there is then more evaporated from the liot stone than the super- 

 incumbent air can sustain in solution, and the surplus is temporarily 

 deposited in a visible form. And in the wetness of the roads under 

 the condition stated we see, in addition to effect of the trees ia 

 increasing the humidity of the atmosphere by evaporation through 

 the stomates of the leaves, the effect which they bear in oheokiug or 

 preventing such evaporation from the ground. 



The quantity of moisture held suspended in solution in air, or 

 which may be so held, increases with the temperature, and when the 

 sun's rays fall unimpeded upon the road this raises the temperature 

 of the earth and stones ; and these by cnntact and radiation raise the 

 temperature of the incumbent air, which in its turn takes up the 

 moisture with which they were covered : the moisture being evaporate^ 

 in the same way as is a drop of water falling on a hot stove, or as the 

 moisture on a wet handkerchief when this is held to the fire, but less 

 rapidly, as the temperature is not so high. 



The same difference as is seen on the exposed and shaded part of 

 the road may be seen in the ground on the sunny and shaded sides o| 

 a house, only less marked because the shade is less complete, 



I have stated in another volume that, to test the correctness of 

 some statements, I had had occasion to m ike at the Cape of Good 

 Hope, on the effect on the destruction of herbage by fire on the 

 desiccation of the country, Mr W. Blore, M. L.A., Fellow of the 

 Meteorological Society of Loudon, and Secretary of the South African 

 Meteorological Society, made some experiments with the following 

 result : — He sunk two cylindrical jars, of the same size, in the ground 

 to the depth of 4 inches, leavin'^ them projecting an inch above the 

 surface, as a precaution against sand and other matters being blown 

 into them, and covering each with wire gauze to keep out flies, &c. 

 The one was placed where it was partially protected, but not covered 

 by bush, the other was sunlc in a newly cleared plot of ground, 

 measuring about 60 feet in diameter, surrounded by sugar bushes, 

 Protea mellifera Thbg, of a considerable height, and otherwise pro- 

 tected from the prevailing wind by a belt of pine trees, about 120 

 feet distant. 



Into each of these jars was put 20 oz. of water on January 31st, at 

 10 A.M. On February 5th, at 5 p.m., the water remaining ia each 

 ■was carefully measured, and the evaporation was calculated, when 



