TABLE XXXII.-Comparison of the observed sub-surface velocity, at nearly the same depth of river, with the different vertical curves assumed by hydraulic engineers. ST. CLAIR RIVER, DEPTH FROM 43 TO 45 FEET. From these comparisons it will be seen that the curve which approaches nearest to the observed velocities is an ellipse, its major axis vertical, and its minor axis at or just above the surface. M. Racourt placed the vertex of the ellipse a little below the bottom, but I find it agrees best with my observations when at the bottom. The minor axis should, apparently, be at the surface in the slow, and a little above it in fast velocities. The straight lines intersecting at six-tenths of the depth, suggested by M. Defontaine, would, if the bottom velocity was a little increased, be nearer the observations than the ellipse, but, as I have heretofore remarked, I think the bottom velocity ought to be rather diminished than increased beyond that I have assumed. I hope next year to report more fully on this point, as I shall this season study especially the velocities near the bottom and the surface. It has been long known that there was great irregularity of flow in running water, but heretofore there has been no means of ascertaining the amount or duration of these irregularities or pulsations. M. Bazin says, (Hydrauliques, page 22 and 23:) The movement of water in a canal is much more complicated than at first appears. If we observe the velocity of a certain point, we very soon perceive that it is changing continually. These variations are sudden; they oscillate by quick impulses, and are accompanied by little changes of the surface level. These continual fluctuations have been already noticed by many observers, and M. Baumgarten has observed them upon the Garrone by aid of a Woltmann's meter. Appreciable even by such an instrument as a meter, which has to run for a certain number of seconds before giving any result, they are much more so by means of an instrument like the gauging tube of M. Darcy, which measures the velocity by quick impulses exerted during a very short time. They are also mentioned by Mr. James B. Francis in the Lowell Hydraulics Experiments, page 175. Mr. John A. Bailey, who was the engineer of the company who laid the first Cuba telegraph cable, tells me that while lying near the buoys, which were put out near the end of the shore cable, about ten miles from Key West, he has often noticed them in calm time, surging down so as to be almost submerged, and again rising so as to be half way out of water. This would occur every two or three minutes, and when anchored between two of the small keys west of the Florida coast, during a calm night, he says, that the steamer would every few minutes drag back on the cable with such force as to make persons standing on deck almost lose their balance. This seems to prove that these pulsations are found in the Gulf stream as well as in small canals and rivers. I am also informed that the oil wells have similar pulsations, differing only in being regular, the time between maximums being from twenty to forty seconds in different wells. This is best shown in the flowing wells, from which the oil will rush with great velocity for a few seconds, and then barely overflow, or, as in some wells, cease altogether. On the other hand, I have not been able to discover any very marked difference in the flow of the artesian wells at Chicago, where the water rises fifty feet or more above the mouth of the well, though I have not yet been able to put a meter into the current. It has also been noticed that when jets are thrown up from a pipe, communicating with a reservoir of a constant level, that the elevation reached by the jet is never uniform, but oscillates between certain limits, these oscillations not being regular, but occurring at intervals of ten or more seconds. I have found these pulsations in every stream I have had access to, and in the following tables I give a few results. These were obtained by putting a Morse register in the circuit; every revolution of the meter causing a dot to be made on the moving paper. The mean point between each maximum and minimum was then found. The time between each of these mean points was measured by a scale, and the number of dots counted. These pulsations for St. Clair River are given in the following tables, as time and revolutions on the maximum and minimum sides of these points. This method of expressing the pulsations does not give the true maximum and minimum, and I could find no way of putting in figures the record shown on the register papers. TABLE XXXIII.-Pulsations in St. Clair River, 208 feet from base; depth of river 28 feet; wind down 11. 9 miles per hour, August 21, 1868. Revolutions METER 20 FEET BELOW SURFACE. METER FEET BELOW SURFACE. 2.1. |