ELASTIC FORCES OF AQUEOUS VAPOUR. 565 
filled with water. The interposition of the glass plate and water 
was thus observed to produce an absolute deviation in the rays 
of light, which sometimes amounted to half a millimetre ; but 
the relative deviations in the divisions traced upon the wet ba- 
rometer in respect to the marked point upon the dry barometer, 
were always much smaller; these deviations, which are the only 
ones that could affect the observations, never exceeded 0°10 
millimetre ; they were nearly always still smaller, and often too 
small to be worthy of notice. At the same time great care was 
observed in placing the barometers quite vertical and parallel to 
the glass plate. 
The case V V' V" is filled with water, which is kept constantly 
agitated ; a very delicate mercurial thermometer dips into the 
water, and is observed by means of a small telescope, L, placed 
horizontally. 
The observer directs the telescope of the cathetometer to the 
top of the mercurial meniscus in the wet barometer; at the mo- 
ment of observation the motion of the water is stopped, and im- 
mediately recontinued, after reading off the height of the first 
meniscus. Then it is again stopped for a moment, to allow the 
observer to read off the meniscus of the dry barometer. The 
water is thus kept in continual agitation, facilitated by the large 
eapacity of the vessel. 
The observations can be made with great precision at the 
temperature of the surrounding air, and can be repeated as many 
times as is wished, To determine the elastic force of vapour at 
_ more elevated temperatures, a portion of the cold water is drawn 
off by means of a siphon, and replaced by hot water; below the 
bottom of the iron vessel a spirit-lamp is placed, which can be 
brought as near as convenient to the bottom, or by regulating 
the height of the wick, can be so managed as, with constant 
agitation of the water, to secure a perfectly stationary tempera- 
ture throughout. This condition is easily attained after some 
trials, and when the temperature of the water does not exceed 
50° C., it is easy to retain it at a stationary temperature for any 
length of time; the only requisite is, that it be agitated rapidly 
and continually. 
Three or four observations are made at the same temperature 
thus kept stationary, and between each two consecutive obser- 
vations an interval of eight or ten minutes is allowed to elapse. 
_ Sometimes the temperature of the bath is purposely made to 
rise two or three hundredths of a degree during the interval be- 
282 
