METEOROLOGY. 



165 



Metcorolo- 



v-5 7 ' 



PL.TC 



CCCIXXIV. 

 Vif. 9. 



Ft-r, ,. >n 

 for tb 



evident from the following geometrical repre- 

 sentation. 



Let the points A, B, C, D, (Fig. .0,) in the HUP AD, 

 present different temperature*, and the perpendiculars 

 .\ :, BA, Cc, Drf, the quantities of humidity which tlu- 

 air at these temperatures can hold in solution. From 

 6 and c draw the lines ft efand eg parallel to AD then 

 C e and D / will he each equal to B b, and fg to t >c ; 

 also ec and gd will represent t'le increased dissolving 

 po-.ver of the air corresponding to the increased temper- 

 atures BC and CD. But according to the theory, the 

 former increase* in a greater ratio than the latter ; and 

 consequently, if CD be equal to BC, gd will be greater 

 than re. Suppose, now, that equal "portions of air, at 

 the temperature* B and D, and both saturated with 

 moisture, are mixed together ; then the temperature of 

 the mixture will be the mem between B and D, in this 

 case C, and the portion at D, l>y having its tem)>erature 

 raised toC, will be able to dissolve ec more nnii-ture 

 than it did before, while the portion at D, by baring its 

 temperature reduced to C, will dissolve gd less than it 

 did at its original temperature. But it was shewn that 

 g d is greater than c c ; therefore the quantity of mots, 

 lure which the two portions of air held in solution in 

 separate state, is greater than what they are capable of 

 dissolving after they are mixed, and consequently a por- 

 tion of their humidity must be deposited by the mix- 

 ture. This portion is obviously equal to the difference 

 between ec and /, or=# dn 



From the relation which the perpendiculars bear to 

 one another, it is evident that the line parsing through 

 their extremities must be a curve, convex towards AD, 

 and auuming what, from experiment, seems to be near- 

 ly the fact, that the dissolving power increases in a 

 geometrical, while the temperature increases in an 

 arithmetical progression, it will tie a logarithmic curve. 

 The knowledge of this fact enables us to assign, in cer- 

 tain cases, a precise value to the quantity deposited, or 

 g d ec. For, from the nature of the figure, we have 

 gd=Dd Cc 

 =Cc B6 



and subtracting these equations, ftif c= D d S C c-f- 

 B 1= 1) </+ B b t C c But C c being a mean propor- 

 i between B A and D d, C c= v'B A x D </ ; there- 



DtpnitiiA 

 Mjr uk* 



fore * d fc=Dd+Bt '2^til>xl)d. In general, 

 let M, m, represent the quantities of moisture v. 

 two portion* of air, at different temperatures, ran hold 

 in solution, when completely saturated, and let these 

 portions be mixed together in equal quantities, *o tli.it 

 the temperature of the mixture may be the mean of tin- 

 two temperatures, then the quantity deposited will be 

 equal to 



M+s_ av/M~7. 



This quantity, or g -I r c. may also be represented 

 geometrically. Thus, join b d. produce C c till it meet 

 6 d in A, and* draw A i parallel to c f or A D. Then the 

 triangles b e k, kid, are obviously similar and equal, nml 

 e A=i d. Also g i=r A, nnd consequently g rf= A -J- >. i 

 +2 c H. Therefore the quantity deposited, or g d 



air to not 

 Miurt<d. 



But a deposition of moisture may take place, even 

 though the portions of air are not both saturated before 

 intermixture. The quantity deposited will be less in- 

 deed in proportion to the distance of the air from the 

 point of saturation ; but still a deposition will take 

 place so long a* the moisture contained in both po- 

 tt the temperatures B and D, is greater than 8 C c. 



This conclusion may alo be deduced from the table in Meteorolo. 

 the article- I ^ OROMETHY, 3<). Thus let 30 and 87- 

 be the temperatures of the two portions, and let each be v -" p ~<-"~"' 

 1 from the point of saturation ; then the actual quan- 

 <>!'iuoi>turc contained in both will be -O02389-J- 

 901 285 = -003624 grs. or -001912 grs. in one cubic 

 inch. But a cubic inch at 4O (the mean temperature 

 of the mixture.) can dissolve only -001 "82 grs; and 

 consequently -0003 grs. will be deposited by each cubic 

 inch. See f S3 of the same article. 



It appears, from the table now referred to, that when 

 the- temperature increases in arithmetical progression, 

 the dissolving power increases somewhat taster than in 

 a geometrical progression. Thus a cubic inch of air at 

 30, when saturated, holds in solution .00127758 gr. 

 and at 60, -002*6714 gr. If the solvingpower, there- 

 fore, increased in geometrical progression, a cubic inch 

 at 40, when saturated, ought to dissolve only -001775 

 gr. ; but, by the table, it is found to dissolve -001788 

 gr being -000007 gr. greater. This small difference, 

 however, does not at all affect the illustration of the ge- 

 neral principle given above. 



When different portions of the atmosphere are inter- Formation 

 mixed, so as to produce a deposition of moisture, the of doudi - 

 consequence will be, the formation of a cloud. This 

 cloud, from its increased specific gravity, will have a 

 tendency to sink downwards ; and, were the lower stra- 

 ta of the atmosphere of the same temperature with the 

 cloud, and saturated witli moisture, it would continue 

 to descend till it reached the surface of the earth hi the 

 form of rain, or what is commonly called mist. In ge- 

 neral, however, the cloud in its descent passes through 

 warmer region, where the condensed moisture again 

 passes into vapour, and consequently ascends till it 

 reach a temperature sufficiently low to recondense it, 

 when it will begin again to sink. This oscillation will 

 continue till the cloud settles at the point where the 

 temperature and humidity are such, as that the con- 

 densed moisture begins to be dissipated, and which is 

 found, on an average, to be between two and three 

 miles above the surface of the earth. 



When the condensation of moisture is rapid and co- Of Rain. 

 pios, there appears to be no reason to doubt, that rain 

 will be immediately produced. Some philosophers, in- 

 deed, have maintained that such a condensation, how- 

 ever rapid, never can produce any thing but a cloud, 

 and tuat the production of rain is the consequence of 

 certain electrical processes that afterwards take place 

 among the minute particles of which the cloud is com- 

 posed. That electricity is a frequent and a powerful 

 agent in the formation of rain, is extremely probable, 

 but that it is in all cases essential to that phenomenon, 

 mnaas to us to be a gratuitous assumption, which, 

 however ingeniously supported, is still destitute of sa- 

 tisfactory proof. The well known fact, that the rain 

 which accompanies a thunder-storm, is more copious 

 than in any other circumstances, is evidence sufficient 

 that it is frequently modified or increased by the in- 

 fluence of electricity ; but this is all that is certainly 

 known upon the ubject, and we deem it unnecessary, 

 i any particular examination of the 

 hypothesi'. that the particles of moisture in a cloud are 

 .t a distance from each other by certain electrical 

 atm<nhere ; and that it is only by the removal of 

 that the jMrticles unite, and form drpps of rain. 



It i rvidrnt that the mixture of different (xirtions of I'romolcd 

 theat'no-plierc, to which, in the preceding n-m.trks, we b Jpp< le 

 have ascribed the production of rain, must be greatly rrcnt * ot 

 promoted by opposite currents of wind. These cur- * " ' 



