447 



be admitted to exercise a sensible influence, when it is 

 considered how close the observer must be to the instru- 

 ment, and what a considerable length of time is generally 

 necessary for an observation. 



" While upon this subject I may observe that Professor 

 Daniell's rule, — to take as the dew-point the arithmetic mean 

 between the temperatures indicated by the included thermo- 

 meter, at the moment of the deposition of the ring of moisture, 

 and at the instant of its disappearance, appears to me to be 

 erroneous. I have just assigned the reasons which induce me 

 to conclude that the former temperature is (at least in most 

 instances) above the truth ; and it is obvious that the latter 

 must always be on the same side, for evaporation cannot com- 

 mence until the temperature of the ball reaches the point of 

 deposition, and will therefore not be completed until it has 

 actually got above this point. The observed results, there- 

 fore, being both above the true dew-point, so also will be 

 the mean itself. 



" There is one other topic, suggested by a perusal of M. 



KupfFer's note, to which I am anxious to advert. Upon 



ordinary occasions the dew-point formula may be used with- 



P — f 

 out the factor - ^^ -, by which it becomes 



f'zzf'^'0\U{t--t'), 



This is the form to which it is reduced by M. KupfFer; and 

 though not rigorously exact, the error is generally negligible, 

 within tjie ordinary variations 'of atmospheric temperature 

 and pressure. In the case of observations on high mountains, 

 however, it will be indispensable to employ the complete 

 formula, otherwise the calculated dew-point would be appre- 

 ciably lower than the truth. In illustration of this point, I 

 subjoin the particulars of an observation made on the Sugar- 

 loaf mountain in the vicinity of Bray, the dew-point being 

 experimentally determined by Daniell's hygrometer, and 



