68 EFFECTS OF FOEESTS ON HUMIDITY. 



and to the great quantity of humus in that soil, the excess of the 

 quantity of moisture absorbed from the atmosphere over the quantity 

 absorbed by the dry gravelly sandy soil taken from the road. 



The quantities of humus and the quantities of clay, which exercise 

 similar power, though not to the same degree, in each of the parcels, 

 can easily be determined. 



It may be noted that what is now spoken of is not the deposit of 

 dew, which depends on a reduction of temperature. 



Wherever the difference of temperature between day and night is 

 great, and the atmosphere is heavily charged with moisture, the 

 dfeposit of dew is considerable. In England, Luke Howard, with a 

 well-constracted rain gauge, found the deposit of dew in one night 

 towards the end of September equal to '01 inch of water ; and in 

 the last six days of October he obtained '11 of an inch from copious 

 dews and mists. 



In Algeria, at the stations on the coast, writes Steinmitz, in a 

 little volume entitled " Sunshine and Showers,'' after the driest andi 

 hottest days, immediately the sun has set, the soldiers' uniforms 

 become wet with dew, and in a single night the blades of knives in 

 the pocket become rusted. 



Biit apart from this deposit of dew, moisture is absorbed from the 

 atmosphere by the soil. The driest soils contain 13% of moisture. 

 It has been determined by experiments by Schubler that when a 

 soil, which weighs about 1000 tons per acre, is pulverised so as to be 

 freely permeable by the atmosphere, and is exposed to the air after 

 being thoroughly dried, it will absorb in twenty-four hours, if a sandy 

 clay, what is equal to 26 tons of water; if a loamy-clay, 30 ; if a stiff 

 clay, 36 ; if garden mould, 45. 



Cezanne states that, " In the Roussillon, rain is a rarity ; the in- 

 habitants can recall periods extending over a year without notable rain; 

 and that the littoral hillocks behind the buttresses of the Pyrenees 

 are of a very permeable limestone, but the vine flourishes in this 

 region, the leaves cover the rocks with verdure, and the grapes swell 

 with a savoury juice, while along the dusty roads the grasshopper 

 follows the traveller with its cry, the emblem of aridity and drought. 



"It is certainly not from the soil devoid of humidity that the vine 

 drinks its supply, it is from the atmosphere ; and the vine-growers 

 do not deceive themselves — it is not for rain that they look, they 

 know that hope of this would be vain — they cry for the sea breeze, 

 which will perfectly satisfy them. 



