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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 lake of Zumpango, swelled by the extraor. dinary increases of the Rio de Guautitlan, and the influxes from Pachuca, flows over into the lake of San Christobal, with which the Cienegas of Tepejuelo and Tlapanahuiloya communicate. The lake of San Christobal bursts the dike which separates it from the lake of Tezcuco. Lastly, the water of this last basin rises in level from the accumulated influx more than a metret, and traversing the saline grounds of San Lazaro, flows with impetuosity into the streets of Mexico. Such is the general progress of the inundations : they proceed from the north and the north-west.

* 3 feet 9 inches. Trans.

+ 39.371 inches. Trans

The drain or canal, called the Desague Real de Huehuetoca, is destined to prevent any danger from them; but it is certain, however, that from a coincidence of several circumstances, the inundations of the south (avenidas del Sur), on which, unfortunately, the Desague has no influence, may be equally disastrous to the capital. The lakes of Chalco and Xochimilco would overflow, if in a strong eruption of the volcano Popocatepetl, this colossal mountain should suddenly be stripped of its snows. While I was at Guayaquil, on the coast of the province of Quito, in 1802, the cone of Cotopaxi was heated to such a degree by the effect of the volcanic fire, that almost in one night it lost the enormous mass of snow with which it is covered. In the new continent eruptions and great earthquakes are often followed with heavy showers, which last for whole months. With what dangers would not the capital be threatened were these phenomena to take place in the valley of Mexico, under a zone, where, in years by no means humid, the rain which falls amounts to 15 deci. metres *.

The inhabitants of New Spain think that they can perceive something like a constant period - in the number of years which intervene between the great inundations. Experience has proved

* 59 inches, Trans.

that the extraordinary inundations in the valley of Mexico have followed nearly at intervals of 25 years*. Since the arrival of the Spaniards the city has experienced five great inundations, viz. in 1553, under the viceroy Don Luis de Velasco (el Viejo), constable of Castile ; in 1580, under the viceroy Don Martin Enrequez de Alamanza; in 1604, under the viceroy Montesclaros; in 1607, under the viceroy Don Luis de Velasco (el Segundo), Marquis de Salinas ; and in 1629, under the viceroy Marquis de Ceralvo. This last inundation is the only one which has taken place since the opening of the

ce since the opening of the canal of Huehuetoca ; and we shall see liereafter what were the circumstances which produced it. Since the year 1629 there have still been, however, several very alarming swellings of the waters, but the city was preserved by the desague. These seven very rainy years were 1648, 1675, 1707, 1732, 1748, 1772, 1795. Comparing together the foregoing eleven epoquas, we shall find for the period of the fatal recurrence the numbers of 27, 24, 3, 26, 19, 27, 32, 25, 16, 24, and 23; a series which undoubtedly denotes somewhat more regularity than what is ob

* Toaldo pretends to be able to deduce from a great number of observations, that the very rainy years, and consequently the great inundations, return, every 19 years, according to the terms of the cycle of Saros.-Rozier, Journal de Physique, 1783.

served at Lima, in the return of the great earthquakes.

The situation of the capital of Mexico is so much the more dangerous, that the difference of level between the surface of the lake of Tez. cuco and the ground on which the houses are built is every year diminishing. This ground is a fixed plane, particularly since all the streets

of Mexico were paved under the government of · the Count de Revillagigedo; but the bed of the lake of Tezcuco is progressively rising from the mud brought down by the small torrents, which is deposited in the reservoirs into which they flow. To avoid a similar inconvenience, the Venetians turned from their Lagunas the Brenta, the Piave, the Livenza, and other rivers, which formed deposits in them*. If we could rely on the results of a survey executed in the 16th century, we should no doubt find that the Plaza Mayor of Mexico was formerly more than eleven deci. metrest elevated above the level of the lake of Tezcuco, and that the mean level of the lake varies from year to year. If, on the one hand, the humidity of the atmosphere and the sources have diminished in the mountains surrounding the valley, from the destruction of the forests; on the other hand, the cultivation of the land has increased the depositions and the rapidity of

* Andreossy on the Canal of the South, p. 19,

† 43. Trans,

the inundations. General Andreossy, in his excellent work on the canal of Languedoc, has insisted a great deal on these causes, which are common to all climates. Waters which glide over declivities covered with sward, carry much : less of the soil along with them than those which run over loose soil. Now the sward, whether formed from gramina, as in Europe, or small alpine plants, as in Mexico, is only to be preserved in the shade of a forest. The shrubs and underwood oppose also powerful obstacles to the melted snow which runs down the declivities of the mountains. When these declivities are stripped of their vegetation, the streams are less opposed, and more easily unite with the torrents which swell the lakes in the neighbourhood of Mexico.

It is natural enough, that in the order of hydraulical operations undertaken to preserve the capital from the danger of inundation, the system of dikes preceded that of evacuating canals or drains. When the city of Tenochtitlan was inundated to such a degree in 1446 that none of its streets remained dry, Motezuma I. (Huehue Moteuczoma), by advice of Nezahualcojotl, king of Tezcuco, ordered a dike to be constructed of more than 12,000 metres in length, and 20 in breadth*. This dike, partly constructed in the

* 395,369 by 65.6 feet. Trans.

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