the peak of Axusco was determined by a particular hypsometrical method, that is to say, by angles of altitude and azimuths. Having very little time to bestow on this work, I could not flatter myself with bringing together in my map the great number of small Indian villages, with which the banks of the lakes are covered. My principal aim was carefully to ascertain the form of the valley, and to draw up the physical map of a country in which I had measured a great number of elevations by means of the barometer. Circumstances of a favourable nature have enabled me to publish a topographical map from accurate materials. A respectable character, who, by a union rarely to be found in any country, possesses with a 1 rge fortune a strong love for the sciences, M. Don Jose Maria Fagoaga, wished to leave me a precious memorial of his country, in giving me at my departure from Mexico the sketch of a plan of the valley. On his invitation, one of my friends, Don Luis Martin, as good a mineralogist as he is an able engineer, drew up a map from the geodesical operations carried on at different times between the city of Mexico and the village of Huehuetoca, on account of the canals of Tezcuco, San Christobal, and Zumpango. M. Martin joined to these materials a part of the surveys communicated to him by me, in subjecting the delineation to the astronomical observations made by me at the extremities of the valley. The numerous excursions which he had undertaken from a zeal for geology, enabled him to express, with a great deal of truth, the form and the relative height of the mountains which separate the plain of Mexico from those of Tula, Puebla, and Cuernavaca. This map, which I owe to the obliging friendship of M. de Fagoaga, is not, however, the one which is inserted in this work. On examining and comparing it carefully, both with the triangulation of M. Velasquez, the detail of which I possess in an original manuscript, and with the table of astronomical positions ascertained by my observations, I perceived that the eastern bank of the lake of Tezcuco, and the whole northern part of the valley, required considerable alterations. M. Martin himself discovered the inaccuracy of his first sketch, and I engaged M. Oltmanns to reconstruct under his eye the map of the valley from the materials which I had collected. Every point was separately discussed ; and when several surveys disagreed with one another, the mean term was adopted. The following is the chain of the triangles measured by M. Velasquez, in 1773, from the rock of the baths (Peñol de los Baños), near the city of Mexico to the mountain of Sincoque, to the north of Huehuetoca. The angles were measured with an excellent English theodolite of ten inches diameter, provided with two glasses of twenty-eight inches in length. |