Mexico were invited by Martinez to repair to Huehuetoca, to see the water flow from the lake of Zumpango and the Rio de Guautitlan through the gallery. The Marquis de Salinas, the viceroy, according to Zepeda's account, entered more than 2000 metres† on horseback into this subterraneous passage. On the opposite side of the hill of Nochistongo is the Rio de Moctesuma (or Tula), which runs into the Rio de Panuco. From the northern extremity of the socabon, called the Boca de San Gregorio, Martinez carried on an open trench for a direct distance of 8600 metres which conducted the water from the gallery to the small cascade (salto) of the Rio de Tula. From this cascade the water has yet to descend according to my measurement, before it reaches the gulph of Mexico, near the bar of Tampico, nearly 2153 metres, which gives for a length of 323,000 metres§ a mean fall of 63 metres in the 1000.

A subterraneous passage serving for a canal of evacuation, of 6600 metres in length, and an aperture of 10 square metres in section ¶, finish

The water flowed for the first time on the 17th September, 1608.

† 6561 feet. Trans.

28,214 feet. Trans. §1,059,714 feet. Trans.

7056 feet. Trans. ¶ The aperture was said a little before to be 3m, 5 in breadth, and 4m, 2 in height. The square of this is not 10 but 14.7 metres, which correspond to 158 square feet. Trans.

ed in less than a year, is a hydraulical operation which in our times, even in Europe, would draw the attention of engineers. It is only, in fact, since the end of the 17th century, from the example set by the illustrious Francis Andreossy in the canal of Languedoc, that these subterraneous apertures have become common. The canal which joins the Thames with the Severn passes, near Sapperton, for a length of more than 4,000 metres*, through a chain of very elevated mountains. The great subterraneous canal of Bridgewater, which, near Worsley, in the neighbourhood of Manchester, serves for the carriage of coals, has an extent, including its different ramifications, of 19,200 metres† (or 4 common leagues). The canal of Picardy, which is at present going on, ought, according to the first plan, to have a subterraneous navigable passage of 13,700 metres in length, 7 metres in breadth, and 8 metres in height §.

*13,123 feet. Trans. † 62,991 feet. Trans. 45.300 feet in height, 26.965 in breadth, and 26.246 in height. Trans.

Millar and Vazic on canals, 1807. The Georg-Stolten in the Harz, a gallery begun in 1777, and finished in 1800, contains 10,438 metres in length (34,244 feet), and cost 1,600,000 francs (71,172.). Near Forth coal mines are worked for more than 3000 metres (9842 feet) under the sea without being exposed to filtrations. The subterraneous canal of Bridgewater is in length equal to two thirds of the breadth of the Straits of Dover.

They

Scarcely had a part of the water of the valley of Mexico begun to flow towards the Atlantic ocean, when Enrico Martinez was reproached with having dug a gallery neither broad nor durable, nor deep enough to admit the water of the great swellings. The chief engineer (Maestro del Desague) replied, that he had presented several plans, but that the government had chosen the remedy of most prompt execution. In fact, the filtrations and erosions occasioned by the alternate states of humidity and aridity caused the loose earth frequently to crumble down. They were soon compelled to support the roof, which was only composed of alternate strata of marle, and a stiff clay called tepetate. made use at first of wood, by throwing planks across, which rested on pillars; but as resinous wood was not very plentiful in that part of the valley, Martinez substituted masonry in its place. This masonry, if we judge of it from the remains discovered in the obra del consulado, was very well executed; but it was conducted on an erroneous principle. The engineer, in place of fortifying the gallery from top to bottom with a complete vault of an elliptical form (as is done in mines whenever a gallery is cut through loose sand), merely constructed arches, which had no sufficient foundation to rest on. The water, to which too great a fall was given, gradually undermined the lateral walls, and deposited an enor

mous quantity of earth and gravel in the watercourse of the gallery, because no means werę taken to filtrate it, by making it previously pass, for example, through reticulations of petate, exe cuted by the Indians with filaments of the shoots of palm trees. To obviate these inconveniences, Martinez constructed in the gallery at intervals a species of small sluices, which, in opening rapidly, were to clear the passage. This means, however, proved insufficient, and the gallery was stopt up by the perpetual falling in of earth.

From the year 1608 the Mexican engineers began to dispute whether it was proper to enlarge the socabon of Nochistongo, or to finish the walling, or to make an uncovered aperture by taking off the upper part of the vault, or to commence a new gallery farther down, capable of also receiving, besides the waters of the Rio de Guautitlan, and the lake of Zumpango, those of the lake of Tezcuco. The archbishop Don Garcia Guerra, a Dominican, then viceroy, or dered new surveys to be made in 1611 by Alonso de Arias, superintendant of the royal arsenal (armero mayor), and inspector of fortifications (maestro mayor de fortificaciones) a man of probity, who then enjoyed great reputation. Arias seemed to approve of the operations of Martinez, but the viceroy could not fix on any definitive resolution. The court of Madrid, wearied out with these disputes of the engineers, sent to

Mexico in 1614 Adrian Boot, a Dutchman, whose knowledge of hydraulic architecture is extolled in the memoirs of those times preserved in the archives of the viceroyalty. This stranger, recommended to Philip III. by his ambassador at the court of France, held forth again in favour of the Indian system; and he advised the construction of great dikes and well protected mounds of earth around the capital. He was unable, however, to bring about the entire relinquishment of the gallery of Nochistongo till the year 1623. A new viceroy, the Marquis de Guelves, had recently arrived at Mexico; and he had consequently never witnessed the inundations produced by the overflow of the river of Guautitlan. He had the temerity, however, to or der Martinez to stop up the subterraneous passage, and make the water of Zumpango and San Christobal return to the lake of Tezcuco, that he might see if the danger was, in fact, so great as it had been represented to him. This last lake swelled in an extraordinary manner; and the orders were recalled. Martinez recommenced his operations in the gallery, which he continued till the 20th June 1629, when an event occurred, the true causes of which have ever remained secret.

* According to some manuscript memoirs, the 20th Sep tember.