26 PRINCIPLES OF STRATIGRAPHY 



plied. It is precipitated in the form of dew, rain, snow, frost, 

 etc., and is constantly returned to the atmosphere by evaporation. 

 In damp countries near the equator the amount of water vapor in 

 the air may be 3 or even 4 per cent, by volume of the whole air. 

 Thus at Batavia (Java), where the vapor pressure is 21 mm., the 

 amount of water in the air is 2.8 per cent, by volume. The com- 

 position of the air here is: N 76.8%, O 20.4%, H.O 2.8% and 

 a few hundredths of a per cent, of CO.,. At Allahabad (Persia) 

 during the rainy season, 4 per cent, of water vapor by volume has 

 been found in the air (calculated from the vapor pressure, which 

 was 30.7 mm.). In central Europe, even in summer, with a vapor 

 pressure of about 10 mm., the volume of water in the air amounts 

 to only 1.3 per cent. (Hann-40 :/-/.) 



Absolute and Relative Humidity. Expressed in weight of wa- 

 ter, the amount which one cubic foot of air can hold is as follows: 

 one-half grain of water at 0° F., 5 grains at 60° F., and 11 grains 

 at 80° F. At 60° the amount which the air of a room 40X40X 15 

 ft. can hold is nearly 20 pounds, or almost enough to fill a common 

 water pail. The amount of moisture which the air contains is its 

 absolute humidity. The percentage of moisture which the air con- 

 tains at a given temperature, of that which it would contain at that 

 temperature if it were saturated, is its relative humidity, which is 

 50 when the air contains only half the amount which it could con- 

 tain at saturation at that temperature. In the latter state the hu- 

 midity is 100. The average relative humidity of the air on the land 

 is perhaps 60 per cent., and over the ocean about 85 per cent. Be- 

 low 65 per cent, the air becomes dry. In semiarid regions it ranges 

 around 45 per cent., and in desert regions it ranges around 25 or 30 

 per cent. 



At Ghadames (Hann-40:75/), western Tripoli, the relative hu- 

 midity for July is 2/' per cent, and for August 33 per cent. The 

 oasis of Kufra, in the heart of the Libyan desert, has a relative 

 humidity of 2y per cent, in August, 33 per cent, in September, and 

 17 per cent, for the 3 p.m. August mean. The oasis of Kauar, in 

 the heart of the Sahara, has a mean relative humidity of 2^ per 

 cent, in August. In the Punjab district of India, the relative mean 

 humidity in May is 31 per cent, at Lahore, while at Agra, in the 

 northwest province, it is 36 per cent. It must, of course, be borne 

 in mind that the observations made in the inhabitable portions of 

 the desert regions, i. e., the oases, give a much higher relative 

 humidity than occurs in the open desert itself. 



In the semiarid regions of southwestern United States, the fol- 

 lowing humidities occur: 



