lie regardo as tlit moll accurate liygrometcr, 



HYGROMETRY 



It is a happy contain 200 fuch partG j at that of 30, it might iliffolve 



application of the differential thermometer. One of the 400 ; and at 45 on the fame fcale 800. Or if we reckorr 



balls is coated with fine cambric paper, and the paper is by Fahrenheit s div.fions, air abfolutely humid holds, at the 



moiftened vith pure water. Evaporation takes place ; and, hmit of congelation, the hundred-and-hxtieth part of Us 



from -he cold which accompanies this, the liquid falls in the weight of moifture ; at the temperature of 59 degrees, the 



extent of its defccnt is meafured by eightieth part ; at that of 86 degrees, the fortieth part ; 



jppofite Hem. The r u .■ 



the fcale attached. This indicates the degree of cold ; this, 

 ajrain, the extent of evaporation ; and this, lallly, indicates 

 the relative drvnefs of the air, the ev.ipor.itron being pro- 

 portionally greater as the air is more free from moifture. 

 The full effeft is very foon obtained— generally in about 

 two minutes ; and it continues permanent under the fame 

 circumftances, as long as moifture is fupplied to the covered 



ball. r ■ r f • 



This ingenious author obferves, in conlequence ot expen- 

 nients adapted to the purpofe, that the condition of the 

 atinofphere, with regard to drynefs, is extremely variable. 



In our climate, the hygrometer will, during winter, mark 

 from 5 to 25 degrees ; but, in the fummer months, it will 

 generally range between 15 and 55 degrees, and may even 

 rife, on fome particular days, as high as 80 or 90 degrees. 

 In thick fogs, the inftrument ftands almoft at the beginning 

 of the fcale : it commonly falls before rain, and remains low 



during wet weather ; but it mounts powerfully in continued difpofition to ho 

 trafts of clear and warm weather. The greateft drynefs increafed by the i 



at that of 1 1 3 degrees, the twentieth part ; and at that of 

 140 degrees, the tenth part. While the temperature, 

 therefore, advances uniformly in arithmetical progreffion, the 

 diflolving power which this communicates to the air mounts 

 with the accelerating rapidity of a geometrical feries. 



The theory of the precipitation of rain, which he founds on 

 this principle, requires, as he fays, the an"umptioTi, not 

 merely of the mixture of two maftes of air at different 

 temperatures, faturated with moifture, but the continued 

 contaft of two currents of air under thefe conditions ; as 

 it is thus only that a fufficient quantity of water will be 

 furnilhcd to form that copious precipitation which con- 

 llitutes rain. And he adds a calculation, founded on the 

 preceding law, which illuftrates this, and illuflrates exceed- 

 ingly well, the general theory. 



We fhall here take occafion to obferve, that as the capa- 

 city of air for heat is increafed by its /arefaftion, its 

 Id moifture in folution appears to be 

 fame caufe ; and at the fame time the 



yet noticed was at Paris, in the month of September, when removal of prcfFure, which is the confequence of the rare- 

 it reached to 1 20 degrees. But for want of obfervations, fadlion, facilitates the tranfition of water into vapour. From 

 we are totally unacquainted with the real ftate of the air in thefe caufes, if the hygrometer be fufpended within a large 

 the remote and tropical climates. receiver, from which a certain portion of air is quickly 



When the indication of the jiygrometer does not exceed abftrafted, it will fink with rapidity. But the effeft is only 



1 5 degrees, we are dircfted by our feelings to call the air momentary, for the rarefied air foon becomes charged with 



damp ; from 30 to 40 degrees we begin to reckon it dry ; moifture, and confequently ceafes to aft on the wet ball of 



from CO to 60 degrees we fliould account it very dry, and the thermometer. Hence there is every reafon to beheve 



from 70 degrees upwards we might confider it as intenfely that the higher regions of the atmofphere are drier than 



dry. A room is not comfortable, or perhaps wholefome, thofe beneath ; and, without this condition, Mr. Leflie 



if it has lefs than 30 degrees of drynefs ; but tlie atmofphere remarks, our globe muft have been fhrouded in darknefs j 



of a warm occupied apartment will commonly produce an for the cold which reigns in the upper ftrata, would have 



efieft of upwards of 50 degrees. prevented the humidity from afcending to a great elevation, 



Mr. Ledie has invented another inftrument, which gives and have precipitated it in continual fogs or clouds. In the 



indications of the quantity of evaporation from a humid aftual ftate of things, the diminution of temperature, in 



furfacc in a given time — which he has named tlie atmometer. afcending, predominates at firft over the augmented power 



It confiils of a thin ball of porous earthen-ware, two or 

 three inches in diameter, with a fmall neck, to which is 

 cemented a long and rather wide tube, bearing divifions, 

 each of them correfponding to an internal annular feftion, 

 equal to a film of liquid that would cover the outer furface 

 of the ball, to the thicknefs of the thoufandth part of an 

 inch. To the top of the tube is fitted a brafs cap, having 

 a collar of leather, which, after the cavity has been filled 

 with diftilled or boiled water, is fcrewed tight, to prevent 

 the tranfudation of the hquid from being fo copious as to 



of aqueous folution ; and tlie air becomes damper till a 

 heiglit be reached, at which the oppofite effefts of cold and 

 rarefaftion are balanced. Above this, which is the proper 

 region of the clouds, the influence of the rarity of the 

 medium exceeds that of the cold, and the air therefore 

 becomes progrefTively drier, until it melts away into the clear 

 ethereal expanfe. 



On this principle is founded the very beautiful experiment 

 invented by Mr. Lefiie, of caufing water to freeze by the 

 cold produced by its own evaporation. The peculiar 



drop from the ball. Evaporation of the water takes place arrangement for this confifts in placing water in a porous 



from the external furface, the inftrument being fufpended in 

 the air ; and the quantity evaporated in a given time, is 

 difcovered by the defcent of the liqviid in the tube. The 

 ufe of tliis inftrument will require fome dexterity, parti- 

 cularly in adjufting the prefTure of the collar ; and its 

 indications are flow, — but it may often be employed with 

 advantage, where it is of importance to afcertain the aftual 

 rate of exhalation. 



earthen cup, fufpended within the receiver of an air-pump, 

 and placing, at a fhort diftance beneath it, fulphuric acid in a 

 broad fhallow veffel, fo that an extenfive furface of the acid 

 fhall be prefented. On rarefying the air, the evaporation 

 of the water is accelerated, and of courfe the degree of cold 

 produced by that evaporation is increafed. This, however, 

 would foon be checked by the prefence of the watery 

 vapour ; but this the fulphuric acid abforbs, almoft as 



From a variety of obfervations on the fubjeft of evapo- quickly as it is formed ; keeps, therefore, the rarefied air 

 ration he infers, that air has its drynefs doubled at each always dry ; and thus allows the evaporati(m to proceed 

 rife of temperature, anfwering to 15 centefimal degrees, with the fame rapidity. The temperature, therefore, con- 

 Thus, at the freezing point, air is capable of holding a tinues to fall, until the water flioots into cryftals of ice ; and 

 portion of moifture rcprefented by 100 degrees of the hy- even after it is entirely congealed, the ice continues to fuffer 

 grometer; at the temperature of 15 centigrade, it could evaporation, until it wholly difappears. See Leflie's Short 



Account 



