60 
MR. J. J. WATERSTON ON THE PHYSICS OE MEDIA COMPOSED OF 
M. Gay-Lussac’s observations during his balloon ascent, § 34, and also with 
Professor Forbes’s original determination of the nearly constant difference of level 
in the atmosphere that corresponds with the same constant difference in the boiling- 
point of water. 
Suppose, then, we take a small glass vessel, in which there is fixed a delicate ther¬ 
mometric apparatus with a large scale. We heat it to about 100°, and the inside 
being perfectly dry we close it and make it perfectly air-tight by means of a thin, 
finely polished silver capsule. From 100° down to 60° corresponds to an altitude of 
only about 2100 feet, so that for greater elevations or a greater range of the barometer 
we would require to seal it at a higher temperature, or what amounts to the same 
thing, partially exhaust the air while fixing the capsule. In afterwards employing 
this instrument the polished capsule will be a concave mirror so long as the pressure 
of the atmosphere exceeds the tension of the enclosed air. It will become a plane 
reflector when they are in equilibrium, and convex when the tension exceeds the 
atmospheric pressure. 
Now, the image of an object is so different in these three kinds of reflectors, that I 
conceive it will be possible to recognise the point of equilibrium with very considerable 
accuracy, or, perhaps, better by means of an eye-piece adjusted to a certain angle of 
reflection. 
The principal difficulty in such an instrument would probably be in getting the 
temperature of the air and of the thermometer to be perfectly the same at the instant 
of equilibrium. M. Brequet’s metallic spiral thermometer is, perhaps, the best 
adapted, and would make the apparatus very portable. But it is the practical artist 
only who can judge if such an instrument can be made effective. 
The annexed sketch is another form of the apparatus, to be used with a delicate 
mercurial thermometer that may show the temperature of the atmosphere at the 
station. Q is an air thermometer, with a bead of mercury as index, which, before 
observing with it, must be blown into the bulb by putting the finger upon the open 
end, a. 
The air in Q, having now the same temperature and density as the atmosphere 
outside the bead of mercury, is allowed to fall into the stem of the instrument, and 
