56 EFFECTS OF FOEESTS ON HUMIDITT. 



preventing evaporation, by protecting moist ground from tne 

 desiccating effect of wind. 



We often hear of a drying wind ; and many may have remarked 

 that a plear wind does more to dry the roads in spring than does 

 even bright sunshine. Every clear wind is a drying wind ; it is 

 composed of air not surcharged with moisture ; up to the measure of 

 saturation every particle of this air can take up and dissolve 

 additional moisture, and it will do so by simple contact therewith. 

 Were the air stagnant, evaporation might go on slowly, the air in 

 contact with the moisture taking up a portion of it and slowly 

 transferring this to the stratum above, to be in like manner trans- 

 fen'ed to the strata beyOnd ; but moved on, as every particle of the 

 air is, by and with, and in the wind, it imbibes a portion and passes 

 off loaded to let more follow to do the same ; and we see the effect 

 in the rapid drying of ground over which the wind has free course ; 

 while we see the effect of shelter in the continued humidity of the 

 ground wherever it is protected from the wind by a wall, a house, a 

 hedge, or a clump of trees. 



■fo this effect of wind, and the modification of it produced by 

 shelter, reference is made in the before mentioned experiments and 

 observations made by Mr Blore. 



It is stated by Mr Milne Home, chairman of the Council of the 

 Scottish Meteorological Society, in a paper containing suggestions for 

 increasing the supply of spring-water at Malta, and improving the 

 climate of the island, to which I shall afterwards have occasion to 

 refer at greater length, " Halley found that, when water is kept in a 

 room, to which neither sun nor wind had access, the evaporation 

 amounted to 8 inches in a year ; but when exposed to sun and wind, 

 even in the cloudy atmosphere of London, it amounted to 48 inches 

 yearly. More recent and accurate observations make the natural 

 evaporation from soil kept moist not quite so much. Howard of 

 London gives, as the mean of eight years' observations, 30 inches. 

 Dobson of Liverpool found, after four years' observations, a mean 

 annual loss by evaporation of 37 inches ; — the least evaporation 

 being in December, when the temperature was 44°, the greatest in 

 July, when the temperature was 70°. Dalton of Manchester found 



that evaporation there was at the rate of 30 inches; the lowest 



being TOl inches, in December, the highest 40-9, in July. 



" An instructive table was framed by Dalton, showing the numlier 

 of grains of water evaporated, from a given surface and during a 



