H Y G R O M E T R Y. 



595 



Hrgromt. In the above Table, the first column contains the 

 "7- quantity of air, in cubic inches, which was submitted 



*"" v ""* to experiment, after making the necessary corrections 

 for pressure, and expansion by the accession of vapour ; 

 the second column contains the temperature of the 

 moist air, which, as we already remarked, was kept 

 two or three decrees below the temperature of the 

 apartments in which the experiments were performed ; 

 and the two last columns exhibit the quantities of mois- 

 ture in a cubic inch of air, as determined by the Table 

 in 39, as well as by actual experiment. The coinci- 

 dence between the results it sufficiently near to confirm, 

 in the most satisfactory manner, the extreme accuracy 

 both of Mr Dalton's experiments on the force of va- 

 pour, and of Gay Lussac's researches into the relation 

 subsisting between dry air, and the quantity of vapour 

 contained in the space' which it occupies. The greatest 

 deviation occurs in the second experiment ; but it is 

 easily accounted for cm the ground, that the air sub- 

 mitted to trial had only stood three hours over water ; 

 whereas, in all the other cases, it bad been allowed to 

 remain in that situation from ten to twenty-four hours. 

 Hence we may safely conclude, that the Table in 39, 

 deduced from the experiments of Dalton and Gay Lus- 

 sac, exhibits with a degree of accuracy, which cannot 

 derate more than the hundredth part of the whole 

 from the truth, the quantity of moisture in a cubic 

 inch, either of pure vapour, or air perfectly humid, at 

 the various temperatures, from zero to 100" of Fahren- 

 heit It it, therefore, a valuable acquisition to hygro- 

 meter. 



f^tm pro- 93. The Quantity of moisture maintained in the va- 

 fad on porous condition having been thus demonstrated to be 



vtfoor by geldy the effect of caloric modified by pressure, it fol- 

 ***** ^ low, that the elevation of water in the atmosphere, in 



JJJJjJJf the state of vapour, and the subsequent deposition of 

 | {Menus, it, under the various forms in which it again descends 

 tan. M dif- to the surface of the earth, must be ascribed chiefly to 

 changes of temperature. Thus if a cubic inch, having 

 in solution its maximum quantity of vapour at the 

 temperature of 80, were suddenly to be reduced to the 

 temperature of 50 ; since, in the former case, it is ca- 

 pable of holding in solution 00623919 grains of mois- 

 ture, ami in the latter only 002*671* grains, it would 

 deposit by the change of temperature -O0377205 grains, 

 a cubic inch of air, in the same circumstances, at 

 the temperature of SO", should be intermixed with a 

 cubic inch of air at the temperature of 50*, the mean 

 temperature induced would be 65% and the whole 

 quantity of moisture in both, -01001633 grains, or 

 0050US16 in a cubic inch; but at the temperature of 

 . cubic inch is capable of holding in solution only 

 00305897 grain* ; therefore by the intermixture of the 

 two airs, -0009838 grains would be deposited in the 

 liquid state. Nor is it necessary to suppose that the 

 two airs, or to speak more correctly, the spaces which 

 they occupy, are completely saturated with moisture 

 at their respective temperatures, or mixed together in 

 equal quantities : thus, if we take two cubic inches of 

 air at the temperature of 80, whose point of deposition 

 is 75, and three cubic inches at 4O, whose point of 

 deposition is 36 ; the former would contain -0107 44i2 

 grams, and the latter -00468468 grains of moisture ; 

 the intermixed airs, amounting to five cubic inchc., 

 would therefore have together '01542920 grains, or 

 00308580 grains to a cubic inch. The temperature 



from re-eiamhilnf; M. Bioi'i wetkt, that Iba dixover 

 DM bj Cay LOSMC, as w bare stattd, but by Bwt bi 



2X80+3X40 

 induced by the mixture would be 7: or 50 ; 



but, at the temperature of 56, a cubic inch can only 

 hold in solution -00298729 grains; and consequently, 

 in the case we have supposed, -OOOOJ1851 grains of 

 moisture would be deposited by each cubic inch. 



94. The facts which we have stated, are well calcula- 

 ted to elucidate the opinions of Dr Hutton respecting 

 the cause of rain ; but before we develope more fully 

 the beautiful and simple theory, which that distinguish- 

 ed philosopher advanced to explain this very familiar, 

 though hitherto the most perplexing of all meteorological 

 phenomena, it may be proper, in the first place, to take 

 a general view of the hygrometric constitution of the 

 atmosphere : this will both facilitate our researches, and 

 give them a greater decree of precision and generality. 



94. In a preceding part of this article, we endea- Absolute 

 voured to shew, that the mean point of deposition for quantity 

 the globe in general, is about 6 U below the mean tern- ^ L ^ 

 perature ; and though this conclusion, we admit, was nlotpri( ; r jcl 

 drawn by a very limited induction, it seems, as far as columns, 

 observation extends, to be sufficiently conformable to forthedif- 

 the actual state of things, to be received into the class f * nt !" 

 of general facts. We have also shewn (? 34.) that at tu 

 the boiling point, the weight of air is to that of vapour 

 under the same pressure, as .9451476 to .589r61, or as 

 1 to .623537 ;. and since both are affected in their bulk 

 precisely in the same manner, by changes of tempera- 

 ture and pressure, this relation must hold in all cases, 

 so long as the vapour retains the elastic state. Now, 

 if we assume that the mean point of deposition, in the 

 different strata of the atmosphere, has throughout the 

 same relation to the temperature as we have attempted 

 to shew it possesses at the surface of the earth, it will 

 be easy, on this principle, to determine the absoluu- 

 quantity of moisture in the atmospherical columns for 

 the different latitudes, adapted to the standard pressure. 

 All that is necessary for this purpose, is to take the elas- 

 ticity of vapour for a temperature 6 below the mean 

 temperature of the latitude ; to multiply it by .623537, 

 and then divide the product by 30 ; and lastly, to mul- 

 tiply the quotient by 407.4, the height in inches of a 

 column of water, equal to the mean weight of a corre- 

 sponding column of the atmosphere: the product will 

 be the mean height in inches of a column of water, equal 

 to the whole moisture contained in a column of the at- 

 mosphere, having the same base. By proceeding in this 

 manner, we have deduced the followingTable, which ex- 

 hibits the quantity of moisture in the atmosphere, in its 

 nun hygrometric state, from the equator to either pole. 



of thai relation being eiprtuibk by the co-ordinate* of in hyperbola, 

 ntclf. 



4 



