564 REGNAULT ON THE 
maintained by a constant and rapid agitation: as soon as this 
ceases, to allow the observer time to measure the difference of 
level in the two mercurial columns, the separation into layers 
immediately begins and the determinations become inaccurate. 
I made, by means of this method, a pretty large number of de- 
terminations of the elastic force of aqueous vapour between + 10 
and +30 degrees ; temperatures which differed but little in each 
case from the surrounding temperature. I shall not however enu- 
merate them here, as they are identical with those which I shall 
give presently, and which were made with apparatus that I con- 
sider more appropriate. Another series of experiments was made 
with an apparatus consisting of two barometers, the one dry and 
the other wet, without heating the mercurial columns through- 
out their whole extent. 
Two barometers as nearly alike as possible, having 14 milli- 
metres interior diameter, are placed side by side on a board 
P P’ Plate VII., fig. 1. These barometers are passed through 
two round holes a 4 in a case of galvanized sheet-iron V V' V", 
and fixed in these holes by means of caoutchouc. The case, of 
which a horizontal section is given at fig. 3, has at one of its 
sides a rectangular opening E FG H, round which is fixed a 
frame of iron. <A plate of glass with parallel surfaces is fitted 
to this frame by means of a second iron frame EK’ ¥’ G! H’, simi- 
lar to the first, and the two are fastened together by screws. A 
thin strip of sheet caoutchouc cut into the shape of these frames 
is placed between the glass and the frame E F GH, thus ren- 
dering the fastening hermetical. This arrangement allows of the 
glass being removed and replaced with the greatest ease. The 
two barometers dip into the same cup U. The iron case has a 
capacity of about 45 litres; it is placed upon an iron support 
fi We 
To ascertain whether the interposition of the glass and the 
water which filled the case produced a deviation in the luminous 
rays, capable of causing an error in reading off the levels by 
means of the cathetometer, a fine horizontal scratch was made 
previously upon the dry barometer at about the ordinary height 
of the mercury, and centimetre divisions were traced upon the 
wet barometer. The distances from the marked point upon the 
dry barometer to all the consecutive divisions upon the wet ba- 
rometer were then read off,—1st, when the glass was not inter- 
posed ; and 2ndly, when the glass was in its place, and the vessel 
