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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 Stir), on which, unfortunately, the Desague has no influence, maybe 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 decimetres *.
The inhabitants of New Spainthink that they can perceive something like a constant period in the number of years which intervene between the great inundations. Experience has proved that the extraordinary inundations in the valley of Mexico have followed nearly at intervals of 2.5 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 canal of Huehuetoca; and we shall see hereafter 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
* 59 inches. Tram.
* 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 Tezcuco 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, thlT 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 decimetresf elevated above the level of the lake of I 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. ]Q, t 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 MoteuczomaJ, 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 Jake, consisted of a wall of stones and clay, supported on each side by a range of pallisadoes, of which considerable remains are yet to be seen in the plains of San Lazaro. This dike of Motezuma I. was enlarged and repaired after the great inundation in 1498, occasioned by the imprudence of king Ahuitzotl. This prince, as we have already observed, ordered the abundant sources of Huitzilopochco to be conducted into the lake of Tezcuco. He forgot that the lake of Tezcuco, however destitute of water in time of drought, becomes so much the more dangerous in the rainy season, as the number of its supplies is increased. Ahuitzotl ordered Tzotzomatzin, citizen of Coyohuacan, to be put to death, because he had courage enough to predict the danger to which the new aqueduct of Huitzilopochco would expose the capital. Shortly afterwards the young Mexican king very narrowly escaped drowning in his palace. The water increased with such rapidity, that the prince was grievously wounded in the head, while saving himself, by a door which led from the lower apartments to the street.
* 395,369 by 65.6 feet. Trans.