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- 
aw, by which it becomes 
fl =f — 0114 (= t+): 
This is the form to which it is reduced by M. Kupffer; and 
though not rigorously exact, the error is generally negligible, 
within the 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, othefwise 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 
out the factor P 
