RAIN. 



805 



ports a continual evaporation, augmented only by 

 the dryness of the air, and the rapidity of its suc- 

 cessive contacts. Even ploughed land will supply 

 nearly as much moisture to the atmosphere as a 

 sheet of water of equal dimensions. If the whole 

 of the waters, which fall from the heavens, were to 

 return again, the evaporation from the ground might 

 be sufficient alone to maintain the perpetual circu- 

 lation. But more than one third of all the rains 

 and melted snows are carried by the rivers to the 

 to the ocean, which must hence restore this con- 

 tinued waste. The air, in exhaling its watery 

 store, is rendered quite damp; but it may afterwards 

 Income dry, on being transported to a warmer si- 

 tuation. Such is the case with the sea-breeze, parti- 

 cularly in summer. It arrives on the shore cold 

 and moist; but as it advances into the interior of 

 the continent, it grows milder and drier. The 

 moisture deposited by a body of air in minute glo- 

 hules, which remain suspended or subside slowly 

 in the atmosphere, constitutes a cloud. When it 

 comes near us, whether it hovers on the tops of the 

 hills or spreads over the valleys, it receives the 

 name of a fog. The production of rain has, from 

 the earliest times, engaged the attention of philo- 

 sophers; but it was reserved for doctor James Hut- 

 ton, of Edinburgh, to afford the true solution of the 

 problem. His theory of rain was made known in 

 1787; since which period it has been greatly neg- 

 lected by writers upon meteorology, until within a 

 very few years. We shall now give an outline of 

 doctor Hutton's views. Air, in cooling, it is known, 

 has the property of depositing the moisture it con- 

 tains. But how, it may be asked, is it cooled in 

 the free atmosphere, unless by the contact or com- 

 mixture of a colder portion of the same fluid? Now 

 the portion of air which is chilled must, in an equal 

 degree, warm the other. If, in consequence of this 

 mutual change of condition, the former be disposed 

 to resign its moisture, the latter is more inclined to 

 retain it; and, consequently, if such opposite effects 

 were balanced, there could on the whole be no pre- 

 cipitation of moisture. The separation of mois- 

 ture, on the mixing of two masses of damp air at 

 different temperatures, would therefore prove, that 

 the dissolving power of air suffers more diminution 

 from losing part of the combined heat, than it ac- 

 quires augmentation from gaining an equal measure 

 of it ; and, consequently, this power must, under 

 equal accessions of heat, increase more slowly at 

 first than it does afterwards, thus advancing always 

 with accumulated celerity. The quantity of mois- 

 ture which air can hold, thus increases in a much 

 faster ratio than its temperature. This great prin- 

 ciple in the economy of nature was traced by doc- 

 tor Button from indirect experience. It is the sim- 

 plest of the accelerating kind, and perfectly agrees 

 with the law of solution, which the hygrometer has 

 established. Suppose equal bulks of air in a state 

 of saturation, and at the different temperatures of 

 fifteen and forty-five centesimal degrees, were inter- 

 mixed; the compound arising from such union will 

 evidently have the mean temperature of 30. But 

 since, at these temperatures, the one portion held 

 200 parts of humidity, and the other 800, the ag- 

 gregate must contain 1000 parts, or either half of 

 it, 500; at the mean or resulting temperature, how- 

 ever, this portion is only capable of suspending 400 

 parts of humidity, and, consequently, the difference, 

 or 100 parts, amounting to the two hundredth part 

 of the whole weight of air, must be precipitated 

 from the compound mass. In this example, it has 

 been assumed that the portions of differently heated 

 air were saturated with moisture before mixing; 

 but it is only required that they should approximate 



to this condition. The effect, however, of simple 

 commixture would, in most cases, be very small. 

 To explain the actual phenomena, we must have 

 recourse to the mutual operation of a chill and of a 

 warm current driving swiftly in opposite directions, 

 and continually mixing and shifting their surfaces. 

 By this rapidity, a larger volume of the fluid is 

 brought into contact in a given time. Suppose, for 

 instance, the one current to have a temperature of 

 50, and the other that of 70 Fahr.; the blending 

 surfaces will therefore assume the mean tempera- 

 ture of 60. Consequently the two streams throw 

 together 200 and 334.2 parts of moisture, making 

 567.1 parts for the compound, which, at its actual 

 temperature, can hold only 258.6 parts ; the differ- 

 ence, or 8.6 parts, forms the measure of precipita- 

 tion, corresponding to the 2325th of the whole 

 weight of the commixed air. It would thus require 

 a column of air thirty miles in length to furnish, 

 over a given spot, and in the space of an hour, a 

 deposit of moisture equal to the height of an inch. 

 If the sum of the opposite velocities amounted to 

 sixty miles an hour and the intermingling influence 

 extended but to a quarter of an inch at the grazing 

 surfaces, but there would still, on this supposition, 

 be produced, in the same time, a fall of rain reach- 

 ing to half an inch in altitude. These quantities 

 come within the limits of probability, and agree 

 sufficiently with experience and observation. But 

 in the higher temperatures, though the difference 

 of the heat between the opposite strata of air should 

 remain the same, the measure of aqueous precipi- 

 tation is greatly increased. Thus, while the mix- 

 ture of equal masses of air, at the temperatures of 

 40 and 60, is only 6.6, that from a like mixture 

 of 80 and 100 amounts to 19. This result is 

 entirely conformable to observation, for showers 

 are most copious during hot weather and in the 

 tropical climates. The quantity of rain precipitated 

 from the atmosphere thus depends upon a variety 

 of circumstances, on the previous dampness of the 

 commixed portions of the fluid, their difference of 

 heat, the elevation of their mean temperature,- 

 and the extent of the combination which takes 

 place. When the deposition is slow, the very 

 minute aqueous globules remain suspended, and 

 form clouds ; but if it be rapid and copious, those 

 particles conglomerate, and produce, according to 

 the temperature of the medium through which they 

 descend, rain, mist, snow, or hail. The foregoing 

 theory tallies precisely with what we experience 

 in the connexion of rains with the variable nature 

 of the winds. Steady dry weather is always ac- 

 companied by a steady direction of the wind ; 

 whereas, in rainy weather, the winds are unsteady 

 and variable. The heavy rains that fall in India 

 always take place during the shifting of the mon- 

 soons ; and while they last, the winds are always 

 veering. The annual quantity of rain is greatest 

 at the equator, and gradually diminishes as we 

 approach the pole. This will be evident from the 

 following table, showing the annual depth of rain 

 in different latitudes: 



Lat. N. 



Grenada, 12 



Cape Frangoise, 19 46' 



Calcutta, 22 23' 



Rome, 41 54' 



England, 50 to 55 



Petersburg, 59 16' 



Uleaborg 65 1' 



Fall of Rain. 

 126 inches. 

 ' 120 



81 



39 " 



31 



16 



On the contrary, the number of rainy days is small- 

 est at the equator, and increases in proportion to 

 the distance from it. From north latitude 12 to 

 43, the mean number of rainy days is 78 



