654 REGNAULT’S HYGROMETRICAL RESEARCHES. 
ture at which the moist thermometer arrives. I nevertheless 
prefer thermometers with cylindrical bulbs, because they are 
much more susceptible of the variations of temperature which 
occur in the air, and because, for the same mass of mercury, 
they present a much greater surface to the air. 
I have ascertained that the manner of moistening the ther- 
mometer also did not exercise any perceptible influence, pro- 
vided that the quantity of water which reaches the cambric 
surrounding the bulb is sufficient. When this quantity is greater 
than that which is evaporated, and consequently a drop of water 
falls from time to time at the extremity of the bulb, I have still 
observed no sensible difference. It is evident, moreover, that the 
quantity of water which arrives in excess must always be very 
small, otherwise it would not have time to cool by evaporation. 
The longer or shorter course which this water runs on the cotton 
wick from the reservoir to the bulb of the thermometer did not 
seem to me to exercise any perceptible influence, at least within 
the limits which are not exceeded in the ordinary construction 
of the psychrometer. 
First Series of Experiments. 
The thermometrical apparatus employed were a thermometer 
A with a dry bulb, the reservoir of which was 8 millimetres in 
diameter and 30 millimetres in length. 
A wet thermometer B, whose bulb was 5 millimetres in dia- 
meter and 60 millimetres in length ; 
A wet thermometer C; diameter of the bulb, 7 millimetres; 
length, 45 millimetres. 
These thermometers are fixed at the extremity of a board 2 
metres long, the bulbs are 4 decimetres above the board; the 
other extremity of the board is fixed to the balcony of a window 
exposed to the north and situated on the first story. These 
thermometers are in the open air in a large court (large qua- 
drangle of the Collége de France), at a distance of 7 metres above 
the ground ; they are observed with a telescope. By means of 
an aspirator and a long glass tube the air is drawn out ata 
short distance from the thermometers, and this air is made to 
pass through weighed drying tubes. During the flowing of the 
aspirator the indications of the thermometers are regularly in- 
scribed every five minutes. The means are taken which enter 
into the formula of the psychrometer, to calculate the quantities 
