ELASTIC FORCES OF AQUEOUS VAPOUR. 589 
mometer was ascertained, as was the 100th degree by the vapour 
of boiling water, the stem being entirely surrounded by vapour. 
The same thermometer was immediately placed in one of the 
iron tubes of the retort in which water was boiling under the 
ordinary pressure of the atmosphere; a portion of the stem in 
this case extended beyond the cover of the retort. The tempera- 
ture indicated by the thermometer under these circumstances 
was corrected for that portion of the column not immersed, by 
taking as basis the temperature indicated by a small thermometer, 
the reservoir of which was placed at the height of the middle of 
the column ; it was found to be identically the same as that previ- 
ously obtained in the apparatus where the entire stem was im- 
mersed in the vapour. This experiment was not conclusive for 
demonstrating the exactitude of the correction, for one must have 
admitted in that case that the mercurial thermometer, immersed 
in the iron tube, indicated exactly the temperature of the vapour 
in the retort. This last result being demonstrable in a direct 
manner, I did not neglect doing it. For that purpose, I forced 
a portion of the mercury of our thermometer out of the stem, 
and, by inclining the stem, brought it into the reservoir at the top ; 
so that the thermometer being now again immersed in the iron 
tube of the retort, whilst the water was boiling under the ordinary 
pressure of the atmosphere, the mercury remained stationary at 
a height of some millimetres above the cover. I observed with the 
greatest care, by means of the horizontal telescope of a cathe- 
tometer, the division marked upon the thermometer; I then 
plunged the instrument into the apparatus which was used for 
determining the point 100° of the thermometers, and read off 
in the same manner the division at which the column of mercury 
stood. I found it quite impossible to perceive the least difference 
in the temperatures indicated under these different circumstances 
by the thermometer, although 1° of temperature took up six 
divisions of the scale. 
The two preceding experiments together show,—lst, that 
thermometers, under the circumstances in which they are placed 
in the apparatus fig. 7, take the temperature of the vapour ; 2nd, 
that the mode of correction which I adopted for that portion of 
the stem which was not immersed is exact, for the temperature 
of 100°. The maximum of correction required for my ther- 
mometers amounted to 0°35. I have assumed that the same 
mode of correction is applicable to temperatures below 100°. 
