HYGROMETRY. 



583 



temperature at the time be given, we have inserted in 

 the last column the mean temperature of the air when 

 the observations were taken. 



; 



53. From what we have stated respecting the mean 

 minimum temperature, it may be inferred, that since 

 the quantity of moisture in the atmosphere must at all 

 times be nearly equal to the maximum quantity of it, 

 that can be maintained in the vaporous state at that 

 temperature, a depression of a few degrees below it must 

 in general be accompanied with a deposition of mois- 

 ture in tome form or other. Hence we can perceive 

 the reason why the air becomes damper towards even- 

 ing ; and why more rain falls during the night than du- 

 ring the day. 



Baiaboebw- ** The ** w ^V "hi*" evaporation is regulated, in a 

 ^p^, , medium either absolutely dry or partially occupied with 

 hvnuJ'tr of vapour, will enable us to investigate the relation subsist- 

 k air. and i n g between the diminution of temperature induced by 

 ** that process, and the humidity of the ambient air For, 

 J^^ J^" since we have shewn, under the article EVAPORATION, 



more palpable form, the different quantities concerned, Hygrome. 

 by means of a diagram. Let AC, therefore, represent _ lry '_, 

 the temperature of the air, and consequently the tern- gjj^jn,, 

 perature also of the covered thermometer, before mpis- tion O f t h e 

 ture is applied to it ; let the line AT represent the time manner in 

 reckoned from A, the parts AD, DD', &c. being equal which a re- 

 intervals ; then, the bulb being moistened, if DE de- """ 

 note the reduction of temperature produced by evapo- mur e 

 ration in the time AD, and Drf (some part of DE) the tkes place 

 proportional quantity of heat which flows into the co- in a ther- 

 vered ball in the same time ; since the reduction of tern- mometer 

 perature depends upon the constant quantity ^j f, '^^ 

 the bulb of the thermr meter would cool uniformly, and bulb, 

 to an indefinite degree, were it not for the incessant in- PLATE 

 flux of caloric, which being always the same proper- <j| CXXVI. 

 tional part of tke excess of the temperature of the nir *'& * 

 above that of the covered ball, gradually increases until 

 BH (which is the same part of BG that D d is of DE,) 

 becomes equal to DE. The diminution of temperature 

 by evaporation being now exactly counterbalanced by 

 the influx of heat from the contiguous air, the cooling 

 process attains its utmost limits ; and OH represents 

 the difference between the temperature of the air and 

 that of the covered thermometer, which henceforth con. 

 tinues stationary. 



56. It must be evident, however, from the view we 

 have taken of the cooling process, that a thermometer 

 with a moistened bulb ought to be reduced through the 

 aame number of degrees in equnl times, and thus reach 

 the maximum of effect in a sudden and abrupt manner, 

 supposition which is neither consistent with the 

 law of continuity, nor conformable to observation : for, 

 though the diminution of temperature is at first nearly 

 uniform, the effect gradually diminishes as. the process 

 advances, and the differences, becoming every instant 

 smaller and smaller, are at last altogether evanescent. 

 The cause of this deviation from the state of things we 

 at first supposed, is to be ascrilx>il to the diminished, 

 evaporation arising from the cooling of the moistened 

 surface ; so that the reduction of temperature in the 

 first interval of time, instead of being represented by 

 E d, the difference between DE and l)d, will properly 

 be represented by de, the minute quantity Ee being so 

 taken, as to represent the diminished effect occasioned 

 by the reduction of temperature already induced. In 

 like manner, the total effect, instead of being accuratc- 



that water, during its transition into the state of vapour, ly represented by HO, will be represented by II g ; and 

 absorbs 098* of calorie ; and consequently, that the era. the curve \e will thus exhibit the march of the ther- 



poration of about the 900th part of a given portion of 

 that fluid would dcprew the temperature of the whole 

 maw one degree, provided it had no communication 

 with the surrounding bodies ; and moreover, since the 

 quantity of water evaporated, in the same circumstan- 

 ces, i< proportional to^f, it follows that the cold 

 produced by evaporation must be a function of the same 

 quantity. Therefore if D represent the reduction of 

 temperature by evaporation, from the moistened bulb 

 of a thermometer, covered with some bibulous sub* 

 stance ; /, the entire force of vapour for the tempera- 

 tare of the air< ; an.l /' the force of the vapour actual- 

 ly existing in the atmosphere, we may evidently have 

 an equation of the form, U=m(f t f), in which m is 

 a constant co-efficient to be determined by experiment, 

 and modified afterwards, if necessary, by a correc- 

 tion depending on the diminished temperature of the 

 evaporating surface. 



55. The condition* implied in this equation, will re. 

 ceive considerable illustration, by representing, in a 



\g 



mometer from the beginning to the end of the process. 

 The interval* A d/ <f <f, &c. will still represent equal 

 portions of time ; and the perpendiculars de, de 1 , &c, 

 the reduction of temperature at the end of those times. 

 A tangent to the curve at g will be parallel to AH,- 

 a condition which is necessary to prevent a violation of 

 the law of continuity. 



57. It may be inferred from this graphical delinea- 

 tion, that since the extent of the evaporation, together 

 with the reduction of temperature which it occasions, 

 is diminished by the cooling of the evaporating surface, 

 the expression D = m (/,/') will require some cor- 

 rection ; and as this correction must have a direct re- 

 lation to D, the simplest way of applying it, is to give 

 the equation the form, 



