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taken account of the fact that the ■warming of the trees by the sun has a considerable 

 effect upon the temperature of the atmosphere outside the woods as well as within them." 



Influence of Woodlands upon Springs, Rivers, and Streams, and in Causing 



Droughts. 



Professor Hough says : — " It is a matter of common remark that our streams 

 diminish as the woodlands are cleared away, so as to materially injure the manufacturing 

 interests depending upon hydraulic power, and to require new sources of supply for our 

 State canals and for the use of cities and large towns. Many streams once navigable are 

 now entirely worthless for this use. 



" The mode in which this influence operates will be readily understood when we con- 

 sider the effect of forests upon the humidity and temperature of the air. 



" A deciduous tree during the season when in foliage is constantly drawing from the 

 earth and giving off from its leaves a considerable amount of moisture, and in some cases 

 this amount is very great. This change of state from a fluid to a gaseous condition is a 

 cooling process, and the air near the surface, being screened from the sun and from the 

 winds, becomes by this means so humid that a rank, succulent vegetation springs up and 

 thrives, which in an open fleld would wither and perish in an hour. The air being thus 

 charged with moisture and cooled, does not take up by evaporation the rains which fall, 

 and the soil being morie open readily allows the water from melting snows and from 

 showers to sink into the earth, from whence a portion appears in springs and in the 

 swamps, which give rise to rills and streams. 



" The air at all times holds more or less watery vapour in suspension, and its capa- 

 city for doing so is increased as the temperature is raised, not by a steadily gaining rate, 

 but more rapidly as the heat is increased. There can be no evaporation when the air is 

 saturated with moisture, and no deposit of water in any form until the temperature is 

 reduced to the point of saturation. It is not yet determined as to how far the cooling and 

 moistening influence of a grove may extend. It must depend upon many circumstances, 

 and especially upon the slope of the surface and thfe direction of the winds. The effect is 

 often apparent to the eye from the freshness of the herbage in adjacent fields for many 

 rods in width." 



He also says : — " Woodlands are well adapted to hinder the waters from running off 

 and to favour their passage into the soil. This they do with better effect when they are 

 more densely covered. It is, moreover, certain that the leaves of trees pump up and 

 absorb a large amount of water, and although the soil on which they grow is uncultivated, 

 it is much more susceptible of absorption of rains than bare and uncultivated land. 



"Porests contribute so effectually to the detention and preservation of the waters, 

 that springs in some countries flowing through the year have entirely disappeared after 

 the woods had been burned, nor did they reappear until after the verdure had been 

 restored, their existence being closely dependent upon its presence." 



I will give a quotation on a very important subject, the amount of moisture evapo- 

 rated by leaves of trees :— - 



" The leaves of plants impart by evaporation during the growing season a certain 

 amount of watery vapour to the air. The amount of this evaporation differs, not only in 

 the different kinds of plants, but it also depends in the same plants upon external 

 conditions — the temperature of the air, the intensity of light, and on the amount of 

 moisture in the air and in the soil. The greater the warmth of the air, the more intense 

 the solar light, the drier the air, and the moister the soil, by so much more will plants 

 give off moisture from their leaves, the transpiration under these conditions being more 

 active. In this respect light affects plants to such a degree that even passing clouds will 

 lessen the evaporation. The result of all the observations thus far has been to show that 

 under like circumstances the transpiration is greatest in the direct light of the sun ; that 

 is, less in common daylight, still less in the shade, and least in the night. Risler found 



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