Page:Popular Science Monthly Volume 6.djvu/249

Rh remarks and discussions to render them intelligible.

Again: "It was determined by actual experiment that the greatest velocity of current is at the surface, and the least at the bottom; and that the increase of velocity 'is in the simple ratio of the distance from the bottom.' "

Taking this ratio as true, draw a vertical right line, to any convenient scale of equal parts, to represent the depth of the river at any given place. Then the velocities at different depths would be represented graphically by an inclined right line, the lower extremity coinciding with that of the vertical and the upper at a distance from the upper extremity of the vertical equal to the surface-velocity. Humphreys and Abbot did not find the relation so simple; they demonstrate that "the velocities at different depths below the surface, in a vertical plane, vary as the abscissæ of a parabola, whose axis is parallel to the water-surface," and that "the position of the axis in calm weather is about $3/10$ of the depth below the surface, whatever be the mean velocity of the river." In order to deduce the relation between the velocity and depth, the river-depth was divided into ten equal parts, and the velocity of each was determined by 222 observations; the mean velocity, which was 0.297 D (depth) below the surface, was 3.26 feet; the mean depth 82 feet.

Very many other points of interest are brought out in the Report; such as proving that the curve of surface-velocities is also a parabola, deducing the values of the parameters of these curves, showing how the curves are affected by winds, irregularities of the bottom, etc., etc. The conclusions were deduced from an immense number of observations taken at different times of the year, different stages of high and low river, and at numerous points, by a number of careful and trained observers. In the mathematical discussion of results, the most refined methods were employed.

The current-metre used by M. Révy, although it is stated that he improved it greatly, is open to grave objections, double floats being preferable in point of accuracy in determining the laws regulating the flow of water in river-channels. I have never used any of the patent current-metres, though I have seen and examined them; but I have used the double-floats often enough to be convinced of their utility. I have, however, used an instrument very similar in principle to the current-metre, one of the numerous patent deep-sea leads. This instrument registered depths in fathoms and quarters on an index-wheel turned by the rotation of a propeller, the latter being so arranged as to turn the index-wheel on going down, but, on being drawn up through the water, was thrown out of gear, so as not to unwind the register.

This lead I tested by over one hundred careful soundings, taken at depths ranging from six to fifty fathoms, simultaneous soundings being taken with the patent lead and an ordinary lead-line. The soundings given by the patent lead were so utterly unreliable that I discarded it, and used the ordinary lead-line for the entire work of some pretty extensive hydrographic surveys. To show that this is not merely my own individual opinion about this style of instrument, I will quote, from page 621, Report of the Chief of Engineers for 1870, the criticism of General Abbot upon the same class of instruments:

"In my opinion, founded on a somewhat close study of the subject, instruments of this class are pretty toys, which have contributed more to retard the progress of discovery in the science of river hydraulics than any other one cause. This is due principally to the fact that they register their results in a kind of cipher, to which we can by no means be sure that we possess the key. To translate a given number of revolutions of a submerged wheel into velocity per second, and by this means to detect laws whose existence is denoted only by differences of a few tenths of feet in this velocity, is so delicate an operation that errors in the coefficient have usually masked the laws."

The subject of river hydraulics is quite an intricate one, and not likely to be investigated by people generally. Therefore, an article on this subject in a magazine is apt to be read and its statements accepted with less of questioning than one upon almost any other scientific topic.

I have been a constant reader of