48 MEXICO, CENTRAL AMERICA, WEST INDIES. 



was resolved to run an undero^round tunnel through the sill which confined the 

 flood waters on the north side. The viceroy summoned a vast army of Indian 

 labourers in order to complete the work within a single winter or dry season 

 from the end of November, 1607, to the middle of May, 1608. The Pluehuetoca 

 or Nechistongo gallery, as it was called, had a total length of 9,000 yards, and a 

 mean height of 12 feet ; but it was not arched and the soil gave way. The outlet 

 was completely closed in 1629, when a terrific storm burst over the city, flooding 

 the streets to a depth of 10 feet. All traflSc was carried on by boats, and five years 

 passed before Mexico again stood on dry land. The works had to be resumed, 

 but were carried on without any general plan and even on mutually destructive 

 lines, in one place by underground galleries, in another by open cuttings. The 

 latter system at last prevailed, and in 1789 the great undertaking was com- 

 pleted. 



At several points the channel, excavated between high rocky walls, presents 

 the appearance of some of the boldest cuttings executed by railway engineers in 

 modern times. For a length of about 860 yards the height of the escarpments 

 exceeds 165 feet, and the opening of the passage is more than double as wide. 

 The river Cuautitlan, which discharged into Lake Zumpango a volume of about 

 400 cubic feet per second, was diverted to this desague, or emissary, and the 

 northern lakes also sent their overflow through the same channel. 



But the friable parts of the cutting were frequently eroded, filling its bed 

 with mud and refuse. Hence the works had to be incessantly renewed, and during 

 the revolutionary wars they were abandoned altogether. Then came the great 

 floods of 1866, which threatened to swamp the capital with the swollen con- 

 tents of the northern lakes rushing through breaches in the embankments, and 

 during which the channel rolled down a volume of from 1,050 to 1,100 cubic feet 

 per second. To prevent such a disaster a new emissary was projected, which was 

 intended to carry off the overflow, not only of the northern lakes, but also that of 

 Texcoco. But little more than a beginning was made with the gallery six miles 

 long, by which the waters were to be drained off through the Tequisquiac Mountain, 

 For twelve years all operations were suspended and not resumed till 1881 ; at 

 present there is some prospect of the works being completed in 1893. 



But scientific men in Mexico are far from being of accord on the subject of 

 drainage. According to L. de Belina the important question is not how to drain 

 the "valley," but on the contrar}^, how to increase its humidity. Arid, dusty, 

 and treeless, the surrounding plains must be transformed to a desert unless the 

 running waters issuing from the uplands are husbanded for irrigation purposes, 

 and unless the slopes of the hills be replanted to improve the climate and regulate 

 the annual discharge. 



Climate — Flora — Fauna. 



Taken as a whole the Mexican climate is one of those that present the greatest 

 contrasts in a narrow space. Here the normal climate, as represented by the 

 parallels of latitude, is profoundly modified by the elevation of the land, the aspect 



