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M. Kupffer, to which also I find it impossible to assent. 
He alleges that the dew-point obtained directly by Daniell’s 
hygrometer is always lower than the truth; and he as- 
cribes this to the bad conducting power of glass, by reason 
of which the opposite surfaces of the ball containing the 
thermometer will, while refrigeration is proceeding, have 
different temperatures, so that when the outer surface has a 
dew deposited on it, the temperature of the inner surface, 
and that of the ether in contact with it, are sensibly lower. 
I do not deny that, theoretically speaking, this must be the 
case; but I certainly doubt much whether the cause assigned 
can produce any appreciable effect of the kind attributed to 
it. On the contrary, according to my experience, the ob- 
served is almost invariably higher than the true dew-point. 
Such must inevitably be the case when the ether is 
poured on too rapidly ; for we have thus a local reduction of 
temperature at the surface of the ether in the ball containing 
the thermometer, considerably greater than that indicated 
by the instrument, as 7¢ merely shows the mean temperature 
of the entire column of fluid in which its bulb is immersed. 
In fact, I have frequently observed, under such circum- 
stances, a ring of dew to be formed, for example, at 44°, 
and to disappear subsequently, though the temperature of 
the inner thermometer was kept steadily at this point, or 
even carried lower,—showing clearly that partial deposition 
may take place before the true dew-point is attained. The 
only mode of avoiding this is to pour on the ether very 
slowly, so as to produce such a gradual lowering of the 
included thermometer, that the entire of the ether in which 
it is immersed shall have, at each instant, a temperature which 
may be considered uniform throughout. As another cause 
why the observed dew-point is higher than the true, I may 
mention the augmentaticn of the humidity of the air in the 
vicinity of the instrument, by the pulmonary halitus and 
cutaneous perspiration of the observer; a cause which must 
