The Isothermal Layer of the Atmosphere

Abstract

I HAVE read with much interest the letters on this subject that appeared in NATURE during last February and March, and also the account of the discussion at the, British Association (NATURE, October 1, 1908), and my only excuse for re-opening the question at this late date is that a point seems to have been overlooked which appears capable of explaining the phenomena without any appeal to an isothermal layer. Both in the correspondence and in the discussion several physicists cast doubt on the accuracy of the thermograms, but, so far as I have seen, only Mr. A. L. Rotch, at the British Association, mentioned that his instruments were verified for low temperatures and pressures. The following physical effect on the barographs does not appear to have been mentioned, and I should be glad to know what precautions are taken to eliminate it in practice. Pressures are necessarily registered by aneroids, and it appears to be assumed throughout all these discussions that a lower pressure on an aneroid means a higher altitude, but this is not so. In 1892, when I was a temporary observer in Ben Nevis Observatory, Mr. Edward Whymper visited the district to have some fourteen or fifteen aneroids of various sizes compared with the mercurial barometers at the low-level station, and as soon as possible afterwards at the top of the hill. It was invariably found that the indexes kept on falling after the aneroids had been brought to rest in the observatory. The rate of fall was at first fast, but became slower as time went on, and it depended upon the difference of pressures between the two stations and also upon the time taken in transit from one to the other, being greater for greater differences of pressure and less for longer times of transit. The aneroid would tend to give the true pressure immediately on arrival or after some hours, according as the standardisation had been rapid or slow. The effect is due to a kind of elastic fatigue, and was reversed on returning the aneroids to sea-level.