"This coal shipment from Pittsburg, which in 1844 only amounted to about 2,500,000 bushels per annum, now amounts to about 40,000,000 bushels per annum. I have, in the special report mentioned, referred to the construction of railroads as having affected the business which was formerly carried on the Ohio river during the comparatively low water. The lower the water, the higher the rates of freight and passenger travel, when there was no railroad competition; but now, when the prices on the river during very low water approach the railroad prices, the freight, whenever it can, will of course take the railroad, on account of the saving of time and greater certainty of delivery; and thousands of passengers always prefer the railroad to the river. But in this connection it is proper to note that since 1844 a large local business between various points on the Ohio, both freight and passenger, has gradually sprung up and become important, which scarcely had existence at that time. The population along the river and in the counties in the several States bordering upon it, and tributary to the river business, has wonderfully increased. So that although a portion of the river business has been attracted to the railroad, the business of steamboats, as a whole, independently of the coal trade, has become much greater than it was in 1844. Meanwhile the coal business has more than kept pace with the increase of population and wealth along the Ohio, in consequence of a steadily augmenting demand for the Pittsburg coal on the Mississippi and other western rivers."[83] The method of inland navigation by means of slackwater formed by dams passable by locks was early proposed for the Ohio River after the first experiment made of this method on the Green River, Kentucky, in 1834-36 by Chief Engineer Roberts. The successful operation of this system on the Monongahela and Muskingum Rivers exerted a powerful influence in its favor, and for many years its adoption on the Ohio was urged patiently though unsuccessfully. At last the important matter was advocated with success, and in 1885 the first of a series of locks and movable dams was erected at Davis Island, four and one-half miles below Pittsburg. The work now is rapidly being completed, the plan being to give a minimum depth of six feet of water in the Ohio by means of thirty-eight dams and locks between Pittsburg and the mouth of the Great Miami, below Cincinnati. This form of improvement will of course be extended in time to the mouth of the Ohio. From past experience with dams in the river, the cost of locks is estimated as follows: For an average lock of six hundred feet length and one hundred and ten feet width, with navigable pass of six hundred feet length, and with weirs of two hundred and forty feet available openings, all arranged to provide six feet navigable depth in the shoalest parts of the improved channels of the pools, with an average lift at each dam of seven and two-tenths feet: