RESULTS OF COMPARATIVE OBSERVATIONS: BAVARIA. 237 



Finally, the greater or less amount of moisture contained in the air 

 has great influence upon human health. Dry air has very often an in- 

 jurious effect upon the organs of respiration, and the fact that we breathe 

 easier in forest air is due to the greater relative moisture. In dry air 

 the cutaneous perspiration is increased, and more warmth is taken from 

 the body. 



After these introductory remarks, we will proceed to show the effect 

 that forests have upon the moisture of the air. As the evaporation of 

 water increases with the increased warmth of the air, it follows that the 

 absolute amount of moisture in the air must be greater in summer than 

 in winter, and must amount to more in lower and warmer regions than 

 in those that are higher and colder. We give below the general mean 

 average pressure of vapor for the different seasons of the year: 



Comparison of average absolute moisture, or pressure of vapor, by seasons. 



Forests do not, therefore, during the year, or in the several seasons, 

 exercise much influence upon the absolute moisture of the air. But in 

 saying this we must bear in mind the distinction between absolute and 

 relative moisture. By the former we mean the amount of vapor contained 

 in a given space, as for instance a cubic foot, which, although invisible, 

 has an independent pressure upon the mercury of the barometer. It 

 cannot be directly read upon that iustrument, but is shown by observa- 

 tions upon the psychrometer, or wet and dry bulb thermometers, and is 

 calculated from tables. 



The weight of vapor contained in a given volume of air saturated 

 with moisture increases with the temperature, as is shown by the fol- 

 lowing table : 



Weight of vapor in a cubic foot in saturated atmosphere at different temperatures. 



The higher the temperature, the greater therefore is the capacity of 

 moisture. The relative moisture is its percentage of saturation, dry air 

 being 0, and air perfectly saturated 100. By increasing the heat we 

 therefore reduce the percentage or relative amount of moisture; and, 

 the,absolute moisture being the same, the tendency to precipitation is 

 greater in cool air, and hence more in woodlands than in the open fields. 



According to Rivenot, climates having an average relative moisture 

 below 70 per cent, are denoted drij, and where over this moist. A very 

 dry climate has the percentage 55; moderately dry, 56 to 70; moder- 

 ately moist, from 71 to 8G, and very moist, from 86 to 100. An equal " ab- 

 solute" amount of watery vapor may be relatively moist or dry, accord- 

 ing to the temperature, and every cause which in any way tends to 

 raise or lower the temperature in corresponding degree increases or les- 

 sens the relative moisture. 



