270 EFFECT OF WINDS: SHELTER-BELTS. 



•western prairies this shelter is needed at all seasons, and quite as much 

 in summer from drying southwesterly winds as in winter from the "Wiz- 

 ards" from the north and northwest. 



With respect to the influence of forests upon temperature, otherwise 

 than by protecting winter snows, obstructing the sweep of winds, and 

 maintaining humidity of climate, some have endeavored to show that a 

 degree of vital heat may be maintained through the action of vegetable 

 organic life. We do not regard this as proven, beyond the effects due 

 to condensation, evaporation, and other changes that may imply the evo- 

 lution or absorption of heat. 



Trees are poor conductors of heat, and come but slowly to an equi- 

 librium of temperature with the surrounding air. While the maximum 

 temperature of the air is usually at about 2 or 3 o'clock p. m., that of 

 the tree is much later. They therefore, in some degree, carry the 

 warmth of the day into the night, and the coolness of night into the 

 early hours of morning. 



WINDS : IMPORTANCE OF SCREENS, WIND-BREAKS, AND SHELTER-BELTS. 



These prove an important agency in the distribution of rains, by bring- 

 ing humid air from the sea or from over other areas where evaporated. 

 They may also prove injurious by their drying effects, as is often felt in 

 the Western and Southwestern States, where drying southwest winds in 

 summer have proved very injurious to agriculture, and especially to fruits. 

 The utility of screens and wind-breaks, is more fully considered in a 

 separate article upon this subject. It is observed in all countries, that 

 coutineutal winds are drying in their effects, and the general prevalence 

 of winds form a westerly point in the Atlantic States, so far as these 

 come from across the treeless plains, may doubtless serve to exj)lain the 

 reason why our climate, on the whole, is sensibly drier than in Europe. 

 Our northwesterly and northerly winds are also for the most part cold 

 and dry, while those from an easterly or southerly point, more generally 

 in the Atlantic States, bring rain. The effect of winds upon our agri- 

 cultural interests deserves more particular notice. 



The influence of strong winds, which blow from the northwest through 

 the dry season in the region around San Francisco, and are drawn by 

 the ascent of heated currents far into the interior, has been noticed by 

 Dr. J. G. Cooper ^ as having an effect upon the tree-growth, the species 

 being few and the growth small, as compared with places further inland 

 and at greater elevations. In some places the trees were bent almost 

 to the ground, and at a place where the Coast Eange was depressed to 

 a height of 350 feet for a distance of ten miles square, the winds blowing 

 inland had sufficient force to limit the tree-growth to scattered groups 

 on the eastern or sheltered slopes of the hills. Other openings along the 

 coast, known to sailors as " wind-gaps," show the same peculiarity. The 

 effect becomes less with altitude, but dryness has more influence, and 

 where these steady winds prevail the slopes that also receive the sun 

 longest present the most barren aspect, although along a coast exposed 

 to the greatest rain-fall and most fog. The winds, therefore, appear 

 to influence tree-growth by their drying power, and by their coolness, 

 and sometimes by their force, as is observed among the Eocky Mount- 

 ains near the timber-line. 



At a special meeting of the Michigan State Board of Agriculture, held 

 January 8, 18G7, a resolution was passed directing T. T. Lyon and the 

 secretary of the board (Sanford Howard) to present a memorial to the 



' Proceedings of the California Academy of Sciences, March IC, 1874. 



