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 Cortéz 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 Cortéz, 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.
The savans of Europe have shown their profound ignorance of the first principles of canal navigation in taking it for granted that the canals of Mexico were filled with stagnant water, that had "set back" from the stagnant pond of Tezcuco; and that the level of the pond must at all times have been so high as to fill the canals, thus keeping the city in constant danger from any sudden rise in the laguna. But, aside from the rules of canal construction, there is an important sanitary question involved.