Our boatmen continued to paddle all day, and as night approached redoubled their exertions, singing to the stroke of their paddles. I was astonished at their endurance. They kept on until eleven o'clock at night, when we reached San Carlos, having accomplished about thirty-five miles during the day against the current. San Carlos is at the head of the river, where it issues from the great lake of Nicaragua, about one hundred and twenty miles from Greytown. The mean level of the waters of the lake, according to the survey of Colonel O.W. Childs, in 1851, is 107 1/2 feet, so that the river falls on an average a little less than one foot per mile. The height of the lowest pass between the lake and the Pacific is said to be twenty-six feet above the lake, therefore at that point the highest elevation between the two oceans is only about 133 feet; but even allowing that an error of a few feet may be discovered when a thorough survey is made across from sea to sea, there can be no doubt that at this point occurs the lowest pass between the Atlantic and the Pacific in Central America. This fact, and the immense natural reservoir of water near the head of the navigation, point out the route as a practicable one for a ship canal between the two oceans. Instead of cutting a canal from the head of the delta of the San Juan to the sea, as has been proposed, the Colorado branch might be straightened, and dredged to the required depth. Higher up, the Torre, Castillo, and Machuca Rapids form natural dams across the river. These might be raised, locks formed round them, and the water deepened by dredging between them. In this way the great expense of cutting a canal, and the fearful mortality that always arises amongst the labourers when excavations are made in the virgin soil of the tropics, especially in marshy lands, would be greatly lessened between the lake and the Atlantic. Another great advantage would be that the deepening of the river could be effected by steam power, so that it would not be necessary to bring such a multitude of labourers to the isthmus as would be required if a canal were cut from the river; the whole track, moreover, passes through virgin forests rich in inexhaustible supplies of fuel.* (* The commission appointed by the United States Government to examine into the practicability of making a canal across the isthmus reported in favour of the Nicaraguan route, and the work was begun at Greytown in 1889. But after an expenditure of 4,500, 000 dollars, the scheme was abandoned, for political reasons, in favour of the Panama route.)