All the lagunas, canals, and ditches that have been described are navigated by small scows that draw but a few inches of water, which are the medium of an extensive internal commerce. Through the lagunas and canal of Chalco come from Cuatla all the supplies of the products of the hot country for the city and surrounding region. This commerce exceeds the whole foreign trade of the republic.[34] This kind of boat was probably introduced by Cortez, and in this convenient form his thirteen brigantines were probably made; for, had his brigantines been of a larger draught of water, they could not have navigated canals intended only for Indian canoes. One of these vessels, when supplied with a sail, a cannon, and a movable keel or side-board, would be a formidable auxiliary in an assault upon the city at the present day. And if one such scow was placed in the ditch on each side of the southern causeway, as Cortez alleges, it would enable an assailing enemy to present just so much more front as the additional width of two boats would give him. THE CAUSEWAYS AND CANALS. Writers have expressed their surprise at the existence of two navigable canals to each causeway, one on either side, as an immense expenditure of unnecessary labor. The explanation of this is found in the fact that in the construction of a pathway (for Cortez says that it was only 30 feet in width) through wet and marshy ground, a broad ditch is ordinarily made on either side to obtain earth for the embankment, and to keep the water-level permanently below the top of the pathway. So it is, and so it must always have been at Mexico, in order to keep these foot-paths in traveling condition. In the dry season, which is the winter, these broad ditches are covered with floating islands of green "scum;" but in the rainy season, which is the summer, they may be navigated by the shallow Mexican scows. A pathway of earth thirty feet in width could not endure the winds and waves of a navigable lake, or the wear and "swash" of a canal twelve feet deep on either side; and the fact that Cortez navigated the ditches in the rainy season establishes the insignificant size of his famous brigantines. As the level of the surface of the land and the surface of the water at Mexicalzingo, at Mexico, and at the village Tezcuco, does not materially vary now from what it was in the time of Cortez, if we can take for data the foundations of the church built by the Conquistadors at these several places, we shall have to look to another quarter for a supply of water for the city canals, which were sufficiently capacious for canoe navigation. This supply we readily obtain by allowing the waters of the canals Tacubaya and Chalco to pass through the streets of the city in ditches sufficiently large for canoes, instead of passing along the south and east fronts outside. By this hypothesis we obtain a current, a prerequisite to the very idea of a canal, particularly in the streets of a city.