The valley of

(of 25 to the equatorial degree). Tenochtitlan, or Mexico, is a basin surrounded by a circular wall of porphyry mountains of great elevation. This basin, of which the bottom is elevated 2277 metres above the level of the sea, resembles, on a small scale, the vast basin of Bohemia, and (if the comparison is not too bold) the vallies of the Mountains of the Moon, described by MM. Herschel and Schroeter. All the humidity furnished by the Cordilleras which surround the plain of Tenochtitlan is collected in the valley. No river issues out of it, if we except the small brook (aroyo) of Tequisquiac, which, in a ravine of small breadth, traverses the northern chain of the mountains, to throw itself into the Rio de Tula, or Moteuczoma.

The principal supplies of the lakes of the valley of Tenochtitlan are, 1. the rivers of Papalotla, Tezcuco, Teotihuacan, and Tepeyacac (Guada

*7468 feet. Trans.

lupe), which pour their waters into the lake of Tezcuco; 2. the rivers of Pachuca and Guautitlan (Quauhtitlan), which flow into the lake of Zumpango. The latter of these rivers (the Rio de Guautitlan) has the longest course; and its volume of water is more considerable than that of all the other supplies put together.

The Mexican lakes, which are so many natural recipients, in which the torrents deposit the waters of the surrounding mountains, rise by stages, in proportion to their distance from the centre of the valley, or the site of the capital. After the lake of Tezcuco, the city of Mexico is the least elevated point of the whole valley.

According to the very accurate survey of MM. Velasquez and Castera, the Plaza Mayor of Mexico, at the south corner of the viceroy's palace, is one Mexican vara, one foot, and one inch higher than the

* According to the classical work of M. Ciscar (Sobre los nuevos pesos y medidas decimales), the Castilian vara is to the toise 0,5130: 1,1963, and a toise 2,3316 varas. Don Jorge Juan estimated a Castilian vara at three feet of Burgos,

mean level of the lake of Tezcuco *, which again is four varas and eight inches lower than the lake

and every foot of Burgos contains 123 lines two-thirds of the pied du Roi. The court of Madrid ordered in 1783 the corps of sea artillery to make use of the measure of varas, and the corps of land artillery the French toise, a difference of which it would be difficult to point out the utility.-Compendio de Matematicas de Don Francisco Xavier Rovira, tom. iv. p. 57 and 63. The Mexican vara is equal to Om, 839.

* The manuscript materials of which I have availed myself in the compilation of this notice are, 1. the minute plans drawn up in 1802, by orders of the dean of the High Court of Justice (Decano de la Real Audiencia de Mexico), Don Cosme de Mier y Trespalacoios; 2. the memoir presented by Don Juan Di az de la Calle, second secretary of state at Madrid in 1646, to King Philip IV.; 3. The instructions transmitted by the venerable Palafox, bishop of la Puebla and viceroy of New Spain, in 1642, to his successor the viceroy Count de Salvatierra (Marques de Sobroso); 4. a memoir which Cardinal de Lorenzana, then archbishop of Mexico, presented to the viceroy Buccarelli; 5. a notice drawn up by the Tribunal de Cuentas of Mexico; 6. a memoir drawn up by orders of the Count de Revillagigedo; and 7. the Informe de Velusquez. I ought also to mention here the curious work of Zepeda,

of San Christobal, whereof the northern part is called the lake of Xaltocan *. In this northern part, on two small islands, the villages of Xaltocan and Tonanitla are situated. The lake of San Christobal, properly so called, is separated from that of Xaltocan by a very ancient dike which leads to the villages of San Pablo and San Tomas de Chiconautla. The most northern lake of the valley of Mexico, Zumpango (Tzompango), is 10 varas foot 6 inches higher than the mean level of the lake of Tezcuco t. A dike (la Calzada de la Cruz del Rey) divides the lake of Zumpango

Historia del Desague, printed at Mexico. I have twice myself examined the canal of Huehuetoca, once in August 1803, and the second time from the 9th to the 12th January, 1804, in the company of the viceroy Don Jose de Iturrigaray, whose kindness and frankness of procedure towards me I cannot speak in too high terms of. (See note D at the end of this work).

* The elevation of the Plaza Mayor, therefore, above Tezcuco is 47.245 inches, and that of San Christobal 11 feet 8.863 inches. Trans.

† 29 feet 1 inch 888. Trans.

into two basins, of which the most western bears the name of Laguna de Zitlaltepec, and the most eastern the name of Laguna de Coyotepec. The lake of Chalco is at the southern extremity of the valley. It contains the pretty little village of Xico, founded on a small island; and it is separated from the lake of Xochimilco by the Calzada de San Pedro de Tlahua, a narrow dike which runs from Tuliagualco to San Francisco Tlaltengo. The level of the fresh-water lakes of Chalco and Xochimilco is only 1 vara 11 inches higher than the Plaza Mayor of the capital*. I thought that these details might be interesting to civil engineers wishing to form an exact idea of the great canal (Desague) of Huehuetoca.

The difference of elevation of the four great reservoirs of water of the valley of Tenochtitlan was sensibly felt in the great inundations to which the city of Mexico for a long series of ages has been exposed. In all of them the sequence of the phenomena has been uniformly the same. The

* 3 feet 9 inches. Trans.