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To the Editor of The Popular Science Monthly:

N the July number of, there is an article entitled "The Hydraulics of Great Rivers," said to have been mainly derived from an account, in the April number of the Edinburgh Review, of a book called "The Parana, the Uruguay, and the La Plata Estuaries," by M. Révy, a member of the Institute of Civil Engineers of Vienna.

In that article there were so many statements calculated to give an erroneous impression to the reader, that it seemed to me some comment was called for. These statements I will examine in turn:

"At the point near Rosario, where the river is 4,787 feet wide, a series of measurements has been made by M. Révy, which constitutes the largest measurement of a river section yet effected. ... The average depth was 47 1/2 feet, and the greatest 72 feet, while the sectional area measured 184,858 feet."

In the "Report upon the Physics and Hydraulics of the Mississippi River," by Captain Humphreys and Lieutenant Abbot, of the Corps of Topographical Engineers, United States Army, published by Lippincott & Co., in 1861, by authority of the War Department, there are given the dimensions of 93 cross-sections of the Mississippi River, commencing at Columbus, Ky., and extending to Fort St. Philip, some 75 miles below the city of New Orleans. Two sections, one at Osceola, Ark., the other at Randolph, Tenn., measured by Lieutenant Abbot in 1858, are, respectively, 6,880 and 6,080 feet in width, 195,844 and 184,717 square feet in area, the maximum depths being 87 and 117 feet; while four of the other sections of the Mississippi exceed 243,000 square feet in area, which M. Révy gives as the measurement of the same Rosario section during the ordinary flood. The claim, therefore, of "the largest measurement of a river-section yet effected," is hardly a valid one. The language, "largest measurement yet effected," rather gives one the idea that it is a pretty difficult matter to measure the cross-section of a large river, whereas it is a comparatively simple operation. A sounding-party in a boat, and two observers at the extremities of a carefully measured base-line on shore with theodolites taking simultaneous observations on the sounding-boat, are all that is necessary to determine the section with great accuracy.

To quote again: "While, therefore, it is easy to measure the velocity of the surface-current, it is difficult, because of this retardation beneath, to determine the mean velocity or actual flow of the river. This has never been satisfactorily done before. Many experiments, with a view to the accomplishment of this end, have indeed been made by eminent men, but they have failed to establish the relationship between the depth of the stream and the velocity of the flow. M. Révy has established that the velocity of a river is directly proportionate to its depth, diminishing or increasing therewith."

It is indeed true that many eminent men have occupied themselves with this problem; but whether they have failed to establish the relation between the velocity and depth is another question, though they certainly failed to find the relation so extremely simple as that determined by M. Révy. In the Government publication referred to, there is an outline of the history of hydraulics applied to rivers, beginning with the contributions to the subject by Castelli, a pupil of Galileo, in 1628, and extending up to the date of the Report. And, by-the-way, Dr. Thomas Young, a portrait and biographical sketch of whom are in the same July number of, was a contributor to this subject. The numerous formula? of different investigators are given in the Report, all reduced to a uniform system of notation, but, are rather too complicated to be merely copied in a letter, without the