308 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 



The annual average temperature within forests is slightly lower than 

 in the open. The difference is greatest in summer, least in winter. 

 The day temperature is less, the night temperature more, than in the 

 open. In summer, a beech forest is more effective for cooling than fir 

 or spruce. The soil temperature is lower in forests, especially in sum- 

 mer, when the difference may amount to 14° F. The mean annual 

 relative humidity is from 3£ to 10 per cent greater than in the open. 

 Nearly one-fourth of the rainfall is intercepted by the trees and evapo- 

 rated or slowly conducted to the ground. Forests somewhat increase 

 rainfall, especially on high ground. The humus formed from fallen 

 leaves diminishes the evaporation from the soil by more than one-half. 

 The whole effect of forests is to retain and more equably distribute the 

 moisture throughout the year, so that streams flowing from them are 

 not torrential, and not subject to heavy floods, but are kept well and 

 moderately supplied. By the prevention of excessive heating of the 

 soil by the sun, and by the diminution of range of daily temperature 

 and of sudden changes, malarious fevers are reduced. The mitigation of 

 strong winds, of hot sunshine, of blizzards, and intense frosts is favor- 

 able to health, and generally the shelter and amenity of well-distributed 

 woods, copses, and forest trees are of great hygienic and agricultural 

 importance. 



CERTAIN PHYSICAL QUALITIES OP THE ATMOSPHERE. 



It is a law of gases that the volume of a given mass is inversely as 

 the pressure; otherwise stated, the density at a constant temperature 

 is proportional to the pressure. The resistance to compression, then, 

 is proportional to the pressure. Yet the law is not exactly true at 

 various pressures and temperatures. Air follows it very closely. Air 

 and nitrogen are, for pressures up to 20 atmospheres at least, more 

 compressed than if this law were exactly true. Amagat, by a fine 

 series of experiments with a tube of mercury extending about 1,000 feet 

 into a deep coal pit, found that air is slightly more compressed up to a 

 pressure of about 80 atmospheres, and then begins to be somewhat less 

 compressed. At about 400 atmospheres the deviation on the side of 

 less compression is nearly one-fifth of the volume, the value pv, or the 

 pressure multiplied into volume, being 1.1897 compared with the origi- 

 nal unit. For pressure diminished below that of the normal it appears, 

 so far as experiment has hitherto gone, that the value pv is practically 

 constant down to at least one eight-hundredth of an atmosphere. No 

 determination has been fully verified for pressures below one-thousandth 

 of an atmosphere. The air at a height of 90 miles is still sufficiently 

 dense to set meteors on fire by friction, but can not exert more than one 

 three- thousandth of the ordinary pressure, unless, indeed, the atmos- 

 phere be surrounded by some lighter gas. Both air and meteor are at 

 a temperature below —180° C. before contact takes place. The experi- 

 mental difficulties of ascertaining the values at these low pressures are 

 exceedingly great. 



