annual fall of water is equal to the full capacity of evaporation. The valley of Mexico is a very small one over which to dispose of the mass of water that the mountain-torrents in summer and the tropical rains pour into it, and with the small margin of six and a half feet for rising and falling, the city must have been in constant jeopardy. Still the floods have been much less frequent than would have been supposed, fully demonstrating the great uniformity in the fall of water in the Mexican season of rain. When a water-spout occurred in the Chalco in 1446, in the time of the Aztec kings, there was a flood, which probably ran off into the Tezcuco. Under the Spaniards the following floods are enumerated: * the first in 1553; * the second in 1580; * the third in 1604; * the fourth in 1607; * the fifth in 1629.
After the flood of 1607, the tunnel of Huehuetoca was undertaken, and constructed in eleven months, for the purpose of letting out of the valley the waters of the River Guautitlan, so as to prevent it from falling into Tezcuco or flooding the city. For those times it was a great work, but we should say now that it was poorly engineered and badly managed, and not worthy the notice it has received in books on Mexico. Since that time, the great inundation of 1629 occurred while the mouth of the tunnel was closed. After that time, the Spaniards, instead of building inside of the tunnel an elliptical tube, actually, by a hundred years of misapplied labor, turned the tunnel into an open cut.
THE MAP OF CORTÉZ.
Cortéz furnished a map to illustrate his description. This map has the same defect as his narrative; that is, it was untrue at the time he made it. In order to bring Tezcuco about the city, he places the village of that