Obsen^ations on tfte Hygiomtter. Tt'7 



vation has suggested a modification of Professor Leslie's theory, 

 which will afterwards be. proposed. 



Dr Anderson, taking a different view of the subject from 

 Professor Leslie, considers D as nearly proportional to the quan- 

 tity of water evaporated under the same circumstances, which 



quantity varies as^-^f, where/' = ^^^'^ ^ ^/ X ff = the ac- 

 tually existing force of vapour. Dr Anderson's formula de- 

 duced from his theory is 



»^* ~~ 36 — tS D ~*^ ^^^ 



This equation, like those derived from Professor Leslie's 

 theory, seems to deviate most from experiment at high tem- 

 peratures; but the error is of an opposite kind, the calculated 

 being often much above the observed dew-point. Indeed, when 

 t is large, it will be found, upon calculation, that Dr Anderson's 

 equation frequently gives t — i" .^^ t — if, which is impossible. 

 There are also one or two points in Dr Anderson's reasoning re- 

 specting this formula, on which the writer would venture, with 

 deference, to make a few observations. 



Having stated that the equation D =: ni (^ — jf') requires 

 a correction depending upon D, Dr Anderson * introduces the 

 consideration of it with the following -remarks : " 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 same number of degrees in equal times, 

 and thus reach the maximum in a sudden and abrupt manner, 

 — a supposition which is neither consistent with the law of con- 

 tinuity nor conformable to observation ; for, although the di- 

 minution 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 ascribed to the diminished 

 evaporation arising from the cooling of the moistened surface." 

 Now, although the evaporation is doubtless diminished from the 

 cause which Dr Anderson assigns, yet the continual retardation 

 of the cooling process seems to arise in but a slight degree from 

 this source ; for, suppose d = the small jx)rtion of heat which 



• Alt. Hygrometry, para. 6«. 



