BAROMETRICAL MEASUREMENT OF HEIGHTS. 



the various co-efficients nearly compensate each other ; partly on the ground that, 

 until a severe test, by means of actual comparative measurements made for the 

 purpose, has shown the expediency of these modifications, it seemed desirable to 

 adhere to the old constants, and thus to preserve a uniformity in the results with the 

 tables of Oltmans, Delcros, Gauss, Baily, and others, which have already been 

 extensively used. The substitution of the co-efficient 0.00260, expressing, accord- 

 ing to Schmidt's computation (Mathem. und Physic. Geogr., II. p. 202), the variation 

 of gravity in latitude, for the value 0.002837, does not sensibly alter the altitudes 

 obtained. 



The close agreement of the determinations furnished by Laplace's formula, in 

 barometrical measurements carefully conducted, made in favorable circumstances, 

 and during the warm season, with those obtained from repeated trigonometrical 

 observations, or by the spirit-level, strongly testifies in favor of its general correct- 

 ness. A few striking examples will suffice to show it. 



The altitude of Mont Blanc, measured by the barometer, by MM. Bravais and 

 Martins, on the 29th of August, 1844, and computed by Delcros, by means of nine 

 corresponding stations situated on all sides of the mountain (see Annuaire Meteoro- 

 logique de France, for 1851, p. 274), was found to be 4810 metres. The altitude of 

 the same point, being the mean of seven of the most elaborate and reliable geodetic 

 measurements, which cost nearly twenty years of labor, is 4809.6 metres. 



For smaller elevations the formula seems to answer equally well. 



The barometrical measurement of Mount Washington, in New Hampshire, by the 

 author, on the 8th and 9th of August, 1851, gave, by Delcros's Tables, for the mean 

 of eight observations, taken at different hours of the day, 5466.7 English feet above 

 Gorham, N. H., 6285.7 above high tide, and 6291.7 feet above the mean level of the 

 ocean in Portland harbor. In August, 1852, W. A. Goodwin, Civil Engineer, start- 

 ing from Gorham Railroad Station, found, by the spirit-level, Mount Washington to 

 be 6285.5 feet above mean tide. In September, 1853, Captain T. J. Cram, of the 

 Topographical Engineers, executed, in behalf of the Coast Survey, a careful measure- 

 ment with the spirit-level, on the same line, for the purpose of testing the various 

 methods of measuring altitudes, and found Mount Washington to be 6293 English 

 feet above the mean level of the ocean. 



In lower latitudes the formula showed equally good results. By a barometrical 

 measurement in July, 1856, the altitude of the highest peak of the Black Mountain, 

 North Carolina, about Lat. 36, was found by the author to be 6701 English feet; 

 and that of the highest Mountain House 5248 feet. In September, 1857, Major 

 T. C. Turner, Chief Engineer of the Morganton Railroad, ran a line of levels from 

 the same point which was used as the lower station for the barometrical measurement, 

 to the top of the highest peak, and found its altitude to be 6711 English feet, and 

 that of the Mountain House 5246 feet. Other points on the line agreed equally well. 



Such an agreement, in so considerable elevations, is all that can be desired. 



These figures show conclusively, that, when the errors which may arise from the 

 great variability of the data furnished by the instruments have been removed by a 

 repetition, in various states of the atmosphere, and by a proper combination of simul- 

 taneous observations at stations not too distant from each other, those which remain 

 and may be attributed to the formula cannot be considerable. But, on the other 



D 34 



