218 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 



of this is that gravitation diminishes regularly with the size of par- 

 ticles of the same substance; but electricity, since it resides on the 

 surface, diminishes at a much slower rate. It is likely that electricity 

 would often cooperate with heat differences in driving the vapor from 

 the surface in an upward direction. Evaporation is increased by low 

 barometric pressure, so that an area of depression to some degree on 

 this account tends to maintain itself. 



By the beautiful law of the diffusion of gases, according to which 

 each gas spreads itself through a space as if that space were a vacuum, 

 subject only to retardation of the rate of diffusion by another gas 

 already permeating the space, vapor diffuses itself through air, not 

 with great rapidity, but so as to produce a fairly equable mixture in 

 the same locality. The molecules of vapor have to encounter thou- 

 sands of molecules of air in every inch and millions in every second 

 of their progress, and if weather depended on diffusion, Avithout the 

 bodily transferences of large quantities of air horizontally and verti- 

 cally owing to perpetually changing distributions of heat, the condi- 

 tions of climate would be extreme and intolerable. 



A very common form of exchange set up where the heat and mois- 

 ture are not excessive by contrast with neighboring masses is by thin 

 streams, filaments, or spirals of lighter vaporous air rising into the 

 upper region, while colder filaments descend toward the earth or sea. 

 This movement occurs under placid conditions, with cloudless sky, and 

 when observed in temperate climates may be taken as a sign of consid- 

 erable stability in the disposition of the atmosphere. 



At other times, also commonly in fine weather, the warmer, lighter 

 strata below break during the daytime into the upper strata by means 

 of small columns, of a good many yards in diameter. These are often 

 capped with rounded cumulus clouds where they attain an elevation 

 and refrigeration beyond their dew point. 



Occasionally, but rarely, the lower air breaks suddenly in a large 

 torrential eddy, which may be several furlongs in diameter, into the 

 upper region. The disturbance may give rise to a cyclone, whirlwind, 

 or tornado. This occurs when the condition is abnormal, the lower 

 strata being very moist and warm and the upper relatively cold and 

 dry, and when from some cause, such as the prevalence of superposed 

 winds, the interchange of differing air volumes has been delayed. The 

 conflict of currents from different directions near the surface may then 

 give rise to an eddy, and this will be a favorable occasion for a rush of 

 light air, as through a chimney, toward the high level. Air flows in 

 from all sides, but can not easily reach the center, owing to the earth's 

 rotation, the onward movement of the whirl, and centrifugal force. In 

 the present writer's opinion, a cyclone may be started or maintained 

 by the strong wind, of 100 miles an hour or more, which often blows at 

 a great elevation in the tropics and neighboring parts. At one observ- 

 atory in the United States a velocity of 180 miles an hour has been 



