INFLUENCE OF WOODLANDS UPON STREAMS. 289 



Some gardeners in the Northern States have a sensible way of retard- 

 ing the tendency to early spring growth in trees that would be otherwise 

 liable to injury from late spring-frosts by packing snow around a tree, 

 and covering it with sawdust, straw, or leaves. This keeps back vege- 

 tation till the danger is over, and delicate exotic trees, not quite hardy 

 enough while young to endure the climate, may be thus accustomed to 

 their location until suflSciently hardy to live alone. It is a practice, of 

 course, limited to ornamental and orchard planting, as a forest-tree that 

 cannot survive without this help might better be given up. 



NATUEAL DEAINAGE FOLLOWING THE CLEAEING OFF OF WOODLANDS. 



A cause of desiccation is sometimes observed to operate in a light 

 gravely soil, after the woodlands have been cut off", which cannot be in- 

 cluded in the class ascribed to climatic changes. It often happens that 

 small perennial streams, having their beds kept up nearly level with 

 the surface by the obstruction of roots and fallen timber, begin to deepen 

 their channels by the action of the current as soon as these obstacles 

 are removed. As a consequence, the land adjacent becomes drained, 

 and soil, before considered damp and even marshy, may become dry 

 and even arid. The most effectual remedy in such cases, where they 

 become inconvenient, would be to set willows abundantly along the 

 banks and near the bottom of the channel, by which obstructions would 

 be created, and the drainage, with proper attention, kept at such depth 

 as might prove most beneficial to the soil. 



INFLUENCE OF WOODLANDS UPON SPRINGS, EIVERS, AND STREAMS, 

 AND IN CAUSING DROUGHTS. 



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 consider the effect of forests upon the humidity and the tem- 

 perature 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 tbis amount is very great. This 

 change of state, from a fluid to a gaseous condition, is a cooling pro- 

 cess, and the air near the surface, being screened from the sun and 

 from the winds, becomes by this means so humid, that a rank succulenu 

 vegetation often springs up and thrives, which in an open field would 

 wither and perish in an hour. The air, being thus charged with moist- 

 ure and cooled, does not take up by evaporation the rains which fall, 

 and the soil, being more 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 vapor in suspension, and 

 its capacity for doing so is increased as the temperature is raised, not 

 by a steadily-gaining rate, but more rapidly as the heat is increased, 

 as is more fully shown on a preceding page of this report. 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 

 19 F 



