426 NOTICES OF THE MEETINGS [April 7, 
Witirsam Roxsureu, M.D., M.R.I. 
On the Cartesian Barometer. 
[The following remarks have been supplied by Dr. Roxsureu in relation to 
a barometer constructed by him and exhibited on this and several preceding 
evenings. ] 
Soon after the discovery of the variations in height of the baro- 
meter, Descartes proposed the following mode of rendering them 
more conspicuous, almost as much so as they are in one filled 
with water alone. He suggested that two tubes should be joined 
to the opposite ends of a short wide cylinder, so as to form one 
straight tube, which, being closed at one end, was to be filled 
with pure water and mercury in such proportions as to allow of 
the two fluids at all pressures meeting in the cylinder. In this, 
the Cartesian barometer, the pressure of the atmosphere is balanced 
by the water and mercury conjointly; but the variations of pres- 
sure are indicated chiefly by movements of the water, as the level 
of the mercury varies little in consequence of the large area of 
the cylinder. The movements of the water and mercury are to 
each other inversely as the areas of the tube and cylinder. The 
scale is that of the common barometer enlarged, as in the wheel 
barometer ; when, therefore, the movements are said to amount 
to so many hundredths of an inch, it is to be understood 2s mean- 
ing that they are equal in value to that height of mercury. The 
scale can be enlarged so as to render movements of 35th of an 
inch visible to the unassisted eye. 
The only records of this instrument that I have seen state that 
the air contained in the water is given off when the pressure is 
removed, and so renders its indications incorrect; also that this 
imperfection is irremediable. This depression, amounting in one 
year, in my first experiment, to only ‘02 of an inch, has led me 
to suppose that the depression which caused the plan to be set 
aside was owing to the force of vapour, which was not so well 
understood at that time as at present; and as many variations of 
pressure are easily seen in this barometer, which would escape 
notice in the mercurial one, and if not attended to give rise to 
error, I think it will prove a valuable addition to a standard baro- 
meter, though never a substitute for one. 
In hopes of getting rid of the air, and of lessening the correc- 
tion required for the force of vapour, I tried several fluids in 
place of pure water. Among these was oil of turpentine; this 
caused a rapid evolution of gas and blackening of the mercury, 
and depressed the column several inches in a few minutes. A 
