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. The present ditches in the middle of the streets, though they have a perceptible current, and a slight infusion of _tequisquite_, are an intolerable nuisance, and have a deleterious effect upon the public health. How much more so must they have been when, from the uncleanly habits of the Indians, they were the common receptacle of all kinds of filth, and were constantly stirred up to their very bottoms by the setting-poles of the navigators? The system of canalling is a system of slack-water navigation, but abhors stagnant water. We come next to the question of the dimensions of these street canals. We know that they were intended only for the navigation of Indian canoes; that two of them, which intersected the causeway of the night retreat, Cortez crossed with his army, all of them climbing down into the canal, wading across, and then climbing up on the other side while loaded with their armor, and fighting all the time against a superior force of the Aztecs; and that Alvarado actually leaped across one of the openings, shows conclusively that the canals could not have more than equaled in breadth the present canal of Chalco. On the hypothesis that Cortez used scows that drew no more water than the scows that at present navigate the canals, his story becomes credible, so far, at least, as the possibility of making the circuit of the city in large boats in a season of rains. TRUTH AGAINST FICTION.