The humidity of the air also materially affects its elasticity, and the hygrometer should, therefore, be conjoined with the thermometer in correcting Iwo metrical observations. But nothing satisfactory has yet been done with regard to that subject. The or dinary hygromters, or rather hygroscopes, are mere toys, and their application to science is altogether hypothetical. A most philosophical coarse has late ly been pursued, by multiplying calculations ground• ed on very loose data, instead of instituting a nice and elaborate train of original experiments.
In the actual state of physical science, it is pre posterous, therefore, to affect any high refinement in the formula for computing barometrical measure ments. The whole operation may be reduced to a very short and easy process. But the simplicity of the calculation win.ld be still greater, if the centesi mal thermometer were generally adopted. It will be sufficiently accurate, till better data are obtained, to assume the expansion of mercury by heat as equal to the 5000th part of its bulk for every - centesi mal degree, while that of air is twenty times great er, being an expansion for each degree of the 250th part of the bulk of this fluid. 1. Car- ] red the length of the mercurial column at the upper station, adding to it the product of its nntkip1iiatioo I into twice the deerence between the degrees on She attached thermometers, the decimal point being shifted four places to the left. 2. Subtract Me logarithm of this corrected length from that of the lower column multiply six, and MOM the decimal point four place; to the rtght I the result is the approximate elevation expressed in English fret. S. Correct this approxi mate elevation, shetutg the decimal point three places back to the right, and multiplying twice the sum of the degrees on the detached thermometers t this product being now added, will give the true elevation.
In illustration of these rules, we shall subjoin some real examples. General Roy, in the month of August 1775, observed the barometer on Caernarvon Quay, at 30,091 inches, the attached centesimal thermo meter indicating 15.7, and the detached 15.6 ; while, on the peak of Snowdon, the barometer fell to 26.409 inches, and the attached and detached thermometers marked respectively 10°,0 and 8°,8. Here twice the
difference of the attached thermometers is and twice the sum of the detached thermometer is 48°.8, which becomes 50.8, when augmented by the fifth part of the mean temperature on that parallel. Now, omitting the lower decimals, the first correction is .00264 x 11.4 = .030, to be added to 26.409. Wherefore, rection to be applied to the upper coluran is : )X 30.6. = .045. Wherefore, And, for the true height, the correction is 3.37 X 50.8 = 171.2, which gives 3340 for the final result.
We shall take another example from the observa tions made b7 Sir George Shuckburgh Evelyn, at the same period, among the mountains of Savoy. This accurate philosopher found the barometer, placed in a cabin near the base of the Mole, and only 672 feet above the surface of the lake of Ge neva, to stand at 28,152 inches, while the attached and detached thermometers indicated 16°.3 and 17°.4; but, another barometer carried to the summit of that lofty insulated mountain, sunk to 24,176 inches, the attached and detached thermometers mark. ing 14°.4 and 13°.4. Wherefore, twice the difference of the degrees on the attached was and twice the sum of the degrees on the detached thermometer was 61°.6. Consequently, the correction to be applied to the higher column was .0024 x 3.8 = .009, which makes it 4.185. Now, To correct this approximate elevation, remove the decimal point three places back, and multiply it by 61°.6, increased by the fifth part of the mean temperature, corresponding to the latitude ; but 3.96 X 644 = 255.4, and 3957.8 + 255.4 = 4218. Hence the summit of the Mole is 4885 feet above the lake of Geneva, or 6083 feet above the level of the Mediterranean Sea.
The last example we shall give is drawn from the observation which Baron Humboldt made among the Andes, near the summit of Chimboraco, the highest spot ever vproaehed by man. This celebrated tra veller found there, that the barometer fell to 14,850 English inches ; the attached thermometer in the tent being at 10°, and the detached in open air be ing 1.6° under Ben. But the same barometer, carried down to the shore of the Pacific Ocean, rose exactly to 30 inches, while both the attached and detached thermometers steed at 25°J. Consequently the cor Now, the difference of the detached thermometer', or 26.9° being doubled and farther increased by 5.8°, the fifth part of the mean temperature at the equator, makes 59°.6 ; the final correction to be ap plied is therefore = 18.24 x 59°.6 = 1087, which gives 19,332 feet for the true elevation observed, or 2140 feet below the summit of Chhnboraco.