The perpendicular height of the Chimborazo is, according to my trigonometrical measurement, 3350 toises (21422 E. feet.) (Recueil d’Observ. Astron., vol. i., Introd. p. lxxii.) This result is intermediate between those given by French and Spanish academicians. The differences depend not on different assumptions for refraction, but on differences in the reduction of the measured base lines to the level of the sea. In the Andes this reduction could only be made by the barometer, and thus every measurement called a trigonometric measurement is also a barometric one, of which the result differs according to the first term in the formula employed. If in chains of mountains of great mass, such as the Andes, we insist on determining the greater part of the whole altitude trigonometrically, measuring from a low and distant point in the plain or nearly at the level of the sea, we can only obtain very small angles of altitude. On the other hand, not only is it difficult to find a convenient base among mountains, but also every step increases the portion of the height which must be determined barometrically. These difficulties have to be encountered by every traveller who selects, among the elevated plains which surround the Andes, the station at which he may execute his geodesical measurements. My measurement of the Chimborazo was made from the plain of Tapia, which is covered with pumice. It is situated to the west of the Rio Chambo, and its elevation, as determined by the barometer, is 1482 toises (9477 E. feet.) The Llanos de Luisa, and still more the plain of Sisgun, which is 1900 toises (12150 E. feet) high, would have given greater angles of altitude; I had prepared everything for making the measurement at the latter station when thick clouds concealed the summit of Chimborazo.