The Isothermal Layer of the Atmosphere

Abstract

IT does not appear to me that the explanation suggested by Mr. Craig is tenable. I will first state that the instruments used in England are calibrated over the whole range of conditions to which they may be exposed by placing them in a glass vessel so that they are completely covered by liquid which has been cooled to the desired temperature by solid CO₂, and then exhausting the air by a pump. Thus the instrument is exposed at the same time to the conditions of pressure and temperature which it will meet with in use. This is done both before and after each ascent, unless, as sometimes happens, such damage is done by the finder as to render the second calibration impossible. On the Continent, at one station at least, and perhaps at most, the pressure is reduced slowly for the express purpose of meeting the point raised by Mr. Craig. In England, and for the same reason, air is generally left in the aneroid box. Very thin metal is used; the box is dried, the faces are squeezed together so that they nearly touch, and the box is then sealed up. The result is that the pressure scale depends on the elasticity of the enclosed air chiefly, and only slightly on the elasticity of the metal. Of course, there is a large correction for temperature which involves extra trouble in the calibration, but, on the whole, I believe this system to be the more accurate.