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rays of tlie sun, which they almost wholly absorb, these clouds are 

 warmed up again in the interior, and budding protuberances are 

 seen, which are especially developed on the upper parts of the cloud. 

 These protuberances are formed and grow so rapidly as to almost sug- 

 gest the presence of a steam generator within every cloud. The ex- 

 ternal parts of the cloud, however, cool very soon by radiation, evap- 

 oration, or dissolution, but especially by their contact with the cold 

 air, into which they continue going. Hence, when the vapors 

 emitted by the cloud reach its periphery, they are cooled at once as if 

 in a condenser; they then take on a rapid movement of descent, 

 which is easily distinguished, and suffer condensation in their lower 

 parts. As the surface of the cloud in contact with the cold all- 

 around it is considerable in proportion to that which receives the 

 influence of the solar rays, the warm ascending currents slacken speed 

 and are extinguished, because the cloudy mass, drawn on by the 

 higher currents, removes from the place where it is formed, or 

 because it stops the rays of the sun and prevents their reaching the 

 ground. There results a more and more complete condensation, and 

 the watery vapor is at last transformed into drops of rain. The con- 

 densation into rain is accelerated and augmented when the mass of 

 cloud rises with great rapidity, especially when it enters abruptly into 

 very cold atmospheric strata. A sudden mixture of the cloud with the 

 air around it takes place then, and sudden and abundant rains result 

 like those which are produced at the instant of thunderstorms. 



The formation and mixture of masses of air of different tempera- 

 tures are effected by ascending currents in zones of restricted ex- 

 tent, but sometimes very numerous. Local showers and thunder- 

 storms are produced in this way. The phenomenon becomes much 

 more important and at the same time extends over vast regions, when 

 it is brought about by the aid of the wind and the larger movements 

 of the atmosphere, and general rains result. 



Babinet, in his Studies on the Sciences of Observation, explains 

 the formation of rain by supposing that when the wind meets an 

 obstacle, it ascends; the moving air cools in rarefying, and deposits 

 its excess of vapor over saturation. This fact, when it occurs, should 

 indeed contribute to the condensation of the vapor contained in the 

 air; but it does not afford an adequate explanation of all rains; for, 

 first, how can it rain on the vast oceans which present no obstacles to 

 cause the air to ascend? It is necessary to suppose that internal 

 movements of the atmosphere intervene in the production of rain. 



Monk, Mason, de Saussure, and many others fix the prime con- 

 dition for the formation of rain in the superposition of two beds of 

 cloud. This assertion, although it is still repeated in a number of 

 treatises on physics, is inexact. A single stratum of cloud — yes, a 



