B 09 OL 08 50 of Göttingen, from a comparison of distant observations on the surface of the globe, proposed a formula, which, with a slight modification, appears to exhibit correctly the temperature of any place at the level of the sea. Leto denote the latitude; and 29 cosq3, or 14 suvers 20, will express, in degrees of the centigrade scale, the medium heat on the coast. But the gradations of climate are more easily conceived by help of a geometrical diagram. From the centre C, draw straight lines to the several degrees of the quadrant, and cutting D the interior semi 40 30 20 C10 610 50 40 30 20 10A 10 15 20 25 E circle; then the radius CA denoting 29°, the perpendiculars from the points of section will intercept segments proportional to the mean temperature expressed on DE. The higher regions are invariably colder than the plains; and I have been able, after a delicate and patient research, to fix the law which connects the decrease of temperature with the altitude. If B and b denote the barometric pressure at the B b łower and upper stations; then will (-) 25 express, on the centigrade scale, the diminution of heat in ascent. Hence, for any given latitude, that precise point of elevation b may be found, at which eternal frost prevails. Put x = B and t the standard temperature; then x2 + .04 tx = 1, which quadratic equation being resolved, gives the relative elasticity of the air at the limit of congelation, whence the corresponding height is determined. From these data the following table has been calculated. This table will facilitate the approximation to the altitude" of any place, which is inferred either from its mean temperature or its depth below the boundary of perpetual congelation. The decrements of heat at equal ascents are not altogether uniform, but advance more rapidly in the higher regions of the atmosphere. At moderate elevations, however, it will be sufficiently near the truth, to assume the law of equable progression, allowing in this climate one degree of cold by Fahrenheit's scale for every ninety yards of ascent, and for every hundred yards in the tropical regions. Thus, the temperatures of the Crawley and Black springs on the ridge of the Pentland hills, were observed by Mr Jardine, where they first issue from the ground, to be 46°.2 and 45°; which, compared with the standard temperature at the same parallel of latitude, would give 567 and 891 feet of elevation above the sea. The real heights found by levelling were respectively 564 and 882; a coincidence most surprising and satisfactory.This ready mode of estimation claims especially the attention of agriculturists. Dr Francis Buchanan informs me, that he found the temperature of a spring at Chitlong, in the Lesser Valley of Népal, to be 14.7 centigrade. But the mean temperature in the parallel of 27°38′ being 220.8, the density of the atmosphere corresponding to difference 8°.1, is .8510, which gives 4500 feet for the corrected altitude. From other observations of the same accurate traveller, we may conclude that Kathmandre, the capital of Népal, is elevated about 2780 feet above the level of the sea. I found myself the temperature of the celebrated fountain of Vaucluse, which gushes with such volume as to form almost immediately a respectable river, to be 13° centigrade, or 2° less than what corresponds to its latitude or 43° 55'. It may hence be inferred, that Vaucluse is 1080 feet above the level of the Mediterranean. The rule stated above for computing the measurements by the barometer, seems to give results somewhat less, on the whole, than those which are obtained from geometrical observations. It would ensure greater accuracy, perhaps, to view the approximate height as answering to a temperature one degree under the point of congelation; and consequent. ly, in applying the last correction, to add unit to the indi cations of the detached thermometers. But the whole subject demands a more thorough investigation. The elasticity of air is affected by moisture as well as heat, although the want of an exact instrument for measuring the former has hitherto prevented its influence from being distinctly noticed. When the hygrometer which I have invented shall become better known to the public, it may not seem presumptuous to expect, in due time, more correct data concerning the modifications of the atmosphere. Yet, after all, in ascertaining the volume of a fluid subject to incessant fluctuation, it would be preposterous to look for that consummate harmony which belongs exclusively to astronomical science; nor can I help regarding the introduction of some late refinements into the formula for measuring heights by the barometer, which would embrace the minutest anomalies of atmospheric pressure, as rather a waste of the powers of calculation. I shall now subjoin a concise table of the most remarkable heights in different parts of the world, expressed in English feet. The altitudes measured by the barometer are marked B, while those derived from geometrical operations, and taken chiefly from the observations of Colonel Mudge, are distin guished by the letter G. Snæ Fiall Jokul, on the north-west point of Iceland, Sulitelma, in Lapland, 4558 G 3950 G 5910 B |