In this survey of the ponds of Mexico, I have drawn upon the experience which has been acquired in the process of evaporation at the extensive salt manufactories of Syracuse and the surrounding villages in Western New York, and also the experience of our engineers Upon the Erie Canal, and the engineers upon the dikes or levees at Sacramento, where the nature of the soil resembles that of Mexico. And I may now conclude this long survey of the canals and lagunas of Mexico, by saying that it is a wise provision of Providence that all bodies of water that have no outlet are found to contain a considerable infusion of salt, otherwise their accumulations of decaying matter would be such that mankind could not live in their vicinity. This valley is an illustration of that truth. Tezcuco, surrounded by barrenness, is not deleterious to life, while the fresh-water lagunas, though continually changing their volume, render Mexico unhealthy in summer by the gases which they exhale from decaying vegetation. ANCIENT POPULATION OF THE VALLEY. I have pretty thoroughly described this small valley, and have also stated how large a portion of it is flooded with surface-water, and how large a portion of this water is infused with salt. In the vicinity of Tacubaya the land is remarkably fertile, and there is good tillable land as the mountains are approached, especially about Chalco on the southeast; but under Indian cultivation, the whole of this valley could have produced sustenance for only an extremely limited population, if the product of the floating gardens and the ducks caught upon the pond should be added. It is totally inadequate to feed the population of Mexico under the vice-kings, 400,000, or its present population of say 300,000; nor could the valley itself be made to sustain one third of this. This valley, it must be recollected, is inclosed on all sides except the north by mountains that exceed 10,000 feet in height, while the commissariat capacity of barbaric tribes is not such as to provide extensive supplies from a distance. Under such circumstances, we should look for an extremely limited population. Yet the most surprising part of the story of the conquest is the enormous population assigned to the numerous large cities which they allege the valley contained. Diaz says, "A series of large towns stretched themselves along the banks of the lake, out of which [the lake] still larger ones rose magnificently above the water." Cortez says that Iztapalapan contained "10,000 families," which would give the town 50,000 inhabitants; "Amaqueruca, 20,000 inhabitants;" "Mexicalzingo, 3000 families," or 15,000 inhabitants; "Ayciaca more than 6000 families;" "Huchilohuchico, 5000 or 6000." The population of Chalco he does not give, nor the population of the very numerous villages whose names he mentions. At the present day there are a few mud huts in nearly every locality named, but not enough in any one instance to merit the name of a village.