Page:Popular Science Monthly Volume 28.djvu/566

550 naturally inclined to adopt the advice of its official experts, but Mr. Eads had faith enough in his plan to propose to do the work at his own expense and wait for his pay until he had demonstrated its success. It was hard to get permission to make even the experimental application of his views thus so liberally proposed; but a bill was finally passed to allow him to attempt the improvement of the South Pass, the smallest of the three, and not the one he had selected, and the depth on the bar of which was only eight feet. The cost of the work was to be five and a quarter million dollars; only half a million was to be paid after a channel twenty feet deep by two hundred feet in width had been secured, another half million after a channel twenty-two feet deep, and other sums on the obtaining of channels of twenty-six and twenty-eight feet depth respectively; but, as a guarantee that the maintenance of the channel should not cost more than one hundred thousand dollars a year, the final million of the whole sum was to be withheld until a channel of thirty feet maximum depth had been kept throughout during twenty years. Congress, however, deeming these terms unnecessarily severe, with remarkable unanimity voted to pay him one and three-quarter million dollars in advance of his contract terms, after he had secured twenty-two feet depth.

The conception of the plan of the jetties was based upon a knowledge of the fact that the Mississippi River is a transporter of solid material, almost all of which is held in suspension by the mechanical effect of the current, and that the quantity of the matter which it is able to carry increases with the square of the velocity. The current of the river is caused by the fall of the water from a higher to a lower level; that is, by the force of gravity. The element which resists the current is the friction of its bed; this friction does not follow the law of solids, but increases or diminishes exactly as the width of the bed or wetted perimeter of its cross-section is increased or diminished: hence, if the stream be contracted, where it is too wide, to one half its width, one half of the frictional resistance will be gone, and the current will be more rapid, and therefore more able to carry a larger load of sediment. This it immediately takes up from its own bed and thus causes a deepening. The result of the application of the jetty system to the South Pass has been a triumphant justification of its author's views.

On the 8th of July, four years after he commenced the work at the jetties, the United States inspecting officer reported that the maximum depth of thirty feet had been secured throughout the jetty channel, and that the least width of the twenty-six-feet channel through the jetties was two hundred feet. The balance due Mr. Eads upon his contract was then paid to him, and the million that was to be held as security for maintenance was considered as earned, and placed at interest for his benefit. The current of the river has maintained this depth ever since. The cost of the jetties was about half of the estimated cost of the proposed canal.