Iflaileg of /IDesico's Drainage, 267 



and magnitude of this work, which will be classed among the grandest 

 achievements of men, and the nearness of its completion, induce me 

 to write this paper, which I hope will give some idea of its scope and 

 purpose. I do not pretend to originality, as my work to some extent 

 has been one of compilation from different monographs, which have 

 appeared from time to time, and from some official publications of the 

 Mexican Government. 



Topographical Conditions of the Valley of Mexico. The Valley of 

 Mexico is an immense basin, of approximately circular shape with one 

 extreme diameter of about sixty miles, completely bounded by high 

 mountains, and having only two or three quite high passes out of it. 

 No water drains out of the basin. The surface of this valley has a mean 

 altitude above the sea of 7413 feet and an area of about 2220 square 

 miles. 



Mountain ranges rise on every side, making a great corral of rock 

 containing dozens of villages and hamlets, with the ancient capital in 

 the centre. In times past the fires of volcanoes licked up the earth, 

 and such fires still live in the mammoth Popocatapetl, from whose 

 great crater sulphur fumes and smoke with jets of flame have poured 

 through the centuries. 



The valley thus hemmed in with solid walls of rock had been an 

 inland sea for many cycles, and during the early existence of man here 

 the salt waters spread over a large extent of the depression. The 

 waters have been gradually lessening by seepage and evaporation, and 

 the Aztec pilgrims coming from the north in the fourteenth century, 

 having received a sign that they were to build their queen-of-the-world 

 city on a small island of the sea, set about building dikes and combat- 

 ing the overflow of the waters. 



Evaporation is so excessive at certain periods of the year that 

 malaria, consequent on drought, was far more dreaded by the inhabit- 

 ants than the periodical floods, and thousands perished annually, so 

 that proper drainage was an absolute necessity for the preservation of 

 health. 



Work done by the Indians. Nearly fifty years before the discov- 

 ery of America, which took place in 1492, Netzahualcoyotl, saw the 

 necessity for a drainage canal, and commenced the work in 1450. 

 He constructed an immense dike to divide the fresh from the salt- 

 water lakes of the valley. The City of Mexico was at this time the 

 centre of the Aztec nation, and was built on floating structures, like 

 rafts, on the water in the numerous islets on the margins of the lakes, 

 so that in the event of the water rising or the city being subjected to a 

 state of siege, the whole city would float. Mexico City now occupies 

 the site of the old Aztec capital. 



The waters of these lakes were liable to disturbances of all kinds; 



