portion of it remains to the present day, and serves to secure the capital from the floods with which it is constantly menaced, we shall describe the whole of this magnificent enterprise at present, though our description will carry us, chronologically, out of the period under consideration, and lead us from the seventeenth to the nineteenth century.
The valley of Mexico is a great basin, which although seven thousand five hundred feet above the level of the sea, and of course subject to constant and rapid evaporation, is yet exceedingly humid for so elevated a region. No stream, except the small arroyo, or rivulet of Tequisquiac, issues from the valley, whilst the rivers Papalotla, Tezcoco, Teotihuacan, Guadalupe, Pachuca and Guautitlan pour into it and form the five lakes of Chalco, Xochimilco, Tezcoco, San Cristoval and Zumpango. "These lakes rise by stages as they approach the northern extremity of the valley; the waters of Tezcoco, being, in their ordinary state, four Mexican varas and eight inches lower than the waters of the lake of San Cristoval, which again, are six varas lower than the waters of the lake Zumpango, which forms the northernmost link of this dangerous chain. The level of Mexico in 1803 was exactly one vara, one foot and one inch above that of the lake of Tezcoco, 2 and, consequently, was nine varas and five inches lower than that of the lake of Zumpango; a disproportion, the effects of which have been more severely felt because the lake of Zumpango receives the tributary streams of the river Guautitlan, whose volume is more considerable than that of all the other rivers which enter the valley combined.
"In the inundations to which this peculiarity in the formation of the valley of Mexico has given rise, a similar succession of events has been always observed. The lake of Zumpango, swollen by the rapid increase of the river Guautitlan during the rainy season, forms a junction with that of San Cristoval, and the waters of the two combined burst the dykes which separate them from the lake of Tezcoco. The waters of this last