REGNAULT’S HYGROMETRICAL RESEARCHES. 639 
at the surface of the liquid where the evaporation occurs, and, 
as liquids are bad conductors of heat, there is always a marked 
difference of temperature between the upper layers of the 
liquid and the lower layers. Thus the deposit of dew always 
commences on a ring which surrounds the surface of the liquid, 
and it is not till afterwards that this deposit extends over the 
whole surface of the bulb. It is necessary therefore to place the 
reservoir of the thermometer in the upper layer of the liquid, 
and to make the dimensions of this reservoir very small, in order 
that the retardation of its temperature over that of the ambient 
liquid may be as little as possible. But in making this reservoir 
of very small dimensions the length of the degree of the ther- 
mometer is much diminished, and there is more uncertainty in 
reading off the instrument. 
In skilful hands, Mr. Daniell’s apparatus may give the tempe- 
rature of the dew-point approximately, but its absolute exact- 
ness cannot be relied on. This apparatus in fact presents several 
inconveniences, which I will enumerate. 
1. The zther presents notable differences of temperature in 
its different layers; the temperature of the upper layer is lower 
than that of the inferior layers. Supposing the thermometer to 
be of extreme delicacy, which is far from being the case, it would 
still only indicate the mean temperature of the layers in which 
its reservoir is immersed. Now this mean temperature may 
differ perceptibly from that on which the first deposit of dew 
depends. The error which may result from this cause is lessened 
by determining a very slow evaporation of zether, at the moment 
when the dew-point is approached ; but we cannot hope to make 
it vanish altogether. 
2. The manipulation requires the long-continued presence of 
the observer near the apparatus: this is very inconvenient, for 
it necessarily influences the hygrometric state of the air and its 
temperature, especially if the observer is obliged to approach 
_ very near to read off the thermometer and to observe the first 
deposit of dew. 
3. The evaporation of a great quantity of zther takes place 
on the bulb B, in a space extremely near to that in which the 
deposit of dew upon the bulb A is determined: it is impossible 
but that this circumstance, and the lowering of the temperature 
which it causes in the neighbouring layers of air, must occasion 
a very sensible change in the hygrometric state of the air. 
