462 PHYSICAL GEOGRAPHY. 



amount of moisture can enter ; but the higher the temperature, the greater the expansion, 

 and the consequent capacity of a volume of the atmosphere to entertain the aqueous vapour. 

 It has been computed, that a cubic mass of air measuring 40 inches each way, at a 

 temperature of 68 Fahrenheit, can contain 252 grains of water, or, taking a cubic mass 

 measuring 20 yards each way, at the same temperature, it will require 252 pounds 

 troy of water to bring it to the point of saturation. Various causes contribute to 

 accelerate or check the process of evaporation, but other things being equal, it will 

 proceed most vigorously, the higher the temperature of the air above that of the surface 

 upon which it acts, and be least active when the two temperatures are the same. The 

 process is materially affected by the state of the air as to dryness and moisture, for water 

 is rapidly evaporated by a stratum of dry air even when the temperature is low, whereas 

 it is conducted tardily, if the atmosphere should contain much vapour, although the 

 temperature may be high. The process also is powerfully promoted by the play of the winds, 

 which bring the atmosphere into immediate and stronger contact with the moist earth and 

 surface waters, and hence every one is familiar with the more rapid drying of the ground 

 after rain, when the air is disturbed, than when it is still. 



By the hygrometer, an instrument employed to ascertain the humidity of the at 

 mosphere, as the name signifies, the measure of moisture, Professor Daniell calculated the 

 average annual amount of evaporation in the vicinity of London to be 23-974 inches, 

 proceeding at the following rate in the different month* : 



Inches. Inches. 



January - - O-413 



February ... 0-733 



March - - 1'488 



April - 2-290 



May - - 3-286 



June ... 3 -760 



July - ... 3-293 



August - ... 3-327 



September ... 2-G20 



October - 1'488 



November ... 0'770 



December - - - 0-516 



Thus, in that locality, evaporation is most active, and the largest amount of water is 

 elevated into the atmosphere, in June, the reverse taking place in January. The annual 

 evaporation from the whole surface of Great Britain is supposed to be equal to 32 inches 

 of water. Now, water extended over the surface of the island to the depth of one inch, 

 would amount to 309,696,038,000,000 cubic inches, which is equal to 1,116,931,402,691 

 imperial gallons, or 4,432,267,441 tons. If we multiply this quantity by 32, we have the 

 prodigious sum of 141,832.558,752 tons of water, ascending in vapour every year from 

 the face of the country. The power of the agency employed in this operation of nature 

 must be tremendous; but equally for its utility does it command attention, as for its 

 wondrous potentiality. For supposing this spontaneous evaporation to cease, the world 

 being deprived of the elements that cause it, the heavens would drop no fatness ; the 

 springs would dry up, and the rivers be exhausted ; the earth would soon be without 

 any vegetation to adorn its surface, or any living creature to inhabit its wilds ; the whole 

 water of the globe accumulated in the ocean would overflow the land, and submerge its 

 now fertile plains. In the temperate zone in general, with a mean temperature of 52^, 

 the annual evaporation is estimated at between 36 and 37 inches ; but in the torrid zone, 

 where the temperature is much higher, the annual evaporation is greater. At Guadaloupe, 

 one of the West India islands, it has been found to amount to 97 inches, and at Cumana, 

 on the north coast of South America, to 100 inches. 



The formation of visible vapours, and their aggregation in masses, take place generally 

 in high regions of the atmosphere under the action of currents, in consequence of a 

 decrease of temperature and a due supply of aqueous elastic vapour being present in those 

 parts where clouds arise. It is easy to perceive that these two conditions, necessary to 



