The question here propounded is one of more practical importance than at first sight it may appear. As the sole object in increasing the velocity of the flow of water in rivers by means of dikes and other appliances is to enable the water to keep suspended, or,not to beg the question,to enable the solid matters to remain suspended in the water, so that they will not deposit in the form of bars, it becomes important to be able to ascertain the precise amount of narrowing and straightening that will secure the desired velocity ; and the question with which we have headed thfis article is certainly important in deciding the question of velocity. To use the words of an able cotemporary, En/jineering, in an article entitled " Fluvial Abrasion," contained in its issue of June 25th, " Velocity alone is needed to convert half a gallon of shot and half a gallon of water into a plumbeous porridge ; indeed, lead, or anything, however heavy, will swim in water if the water only runs sufficiently fast." Engineering goes on to criticise the views of one of its correspondents in regard to this subject, but in our opinion it makes one rather serious mistake, especially as in the article referred to it assumes the role of " philosopher," which it plainly tells its correspondent he is not, although an " able and conscientious engineer." It says, " Mr. Login arrives at what we think must be an erroneous conclusion in deducing from various premises that a certain amount of the energy of running water is absorbed or expended in carrying with it solid matter in suspension. In first putting this matter into motion, power is unquestionably abstracted from the water ; but as soon as uniform flow is established the solid matter flows in obedience to its own gravitation, neither receiving from nor imparting to the water any power whatever. " Its tendency to continue its onward motion is sufficient to overcome gravitation, and as it moves with water of its own velocity, it is in equilibrium *fore and aft,' and thus it moves on with no resistance whatever, unless it be argued that its rate of advance is less than that of the stream. If so, it would drop at once, and the conditions of flow would cease." If this be philosophy, or if the assumption that uniform flow can be at some time fully established be not begging the question, then have we much left to learn in the elements of physics and logic. Let us examine this singular proposition in the light of the following well-known and admitted natural laws. 1st. If two or more forces act upon a body at the same time each of these forces produces the same eff'ect as it it acted alone. 2d. The quantity of motion impart ad to a body by a constant force is in proportion to the time of the application of the force. od. If two forces act simultaneously upon a body in different directions not opposite, it will move in the direction of neither, but in a line between them. A bed of a river is an inclined plane down which the particles of water roll. If it were perfectly smooth there would be no friction and consequently no wear of the bottom, but as the bottoms of all streams are more or less rough, the projections receive the force of the descending water, and, if the current be strong enough, are forced from their beds and either rolled along the bottom, or, if the impetus is strong enough, are carried out on a line nearly parallel to the base of the inclined plane into the stream. When this has taken place gravity acts upon the bf dy, not in a line parallel to the in- clined plane, but in a line perpendicular to its base, which tends to draw the body down to the surface of the inclined plane again by a constant force equal to the difference between the weight of the solid floating body heavier than water, and the weight of an equal bulk of water. What counteracts this tendency during any period of time if not the motion of the water ? And as the overcoming of the action of a constant force implies a constant exertion of some other force, how are we to escape the conclusion that a constant demand is made upon the momentum of the flowing water to keep stones or sand supported in a current? The motion of the water obeys the same laws as those of other bodies rolling down an inclined plane; water being practically homogeneous, no part of it seeks by its own gravity to regain the surface of the plane. But a stone carried along by the force of a stream is constantly making this effort. Something prevents it and that something can be nothing else than the water. If stones, or sand and water, were flowing downward by the force of gravity alone in a vertical line, all would move together (not taking into account resistance of the air) at equal velocities for the same points in the line of descent. But in no other case could this occur. As soon as the stream is inclined the heavier body begins to seek the bottom of the channel, and is only prevented from reaching it by absorbing motion from water flowing more rapidly in a line parallel to the bottom. Thus the stone may be said to receive, the moment it attempts to move toward the bottom, an infinite number of kick from the particles of water which it must check in their flow in order to reach the bottom. It is the game of football repeated ; the ball is kept flying,but it takes power to do it. We have intimated that the speed of solid matters heavier than water must of necessity flow less rapidly in a line parallel to the bottom of the channel than the water which floats them. Many have witnessed the butterfly trick performed by the Japanese jugglers in their exhibitions in this country. It illustrates this truth exactly. Pieces of colored tissue paper are folded to represent butterflies, which, by means of currents of air adroitly produced by fans, are made to float or alight and appear to sustain themselves at the will of the performer. It is a very ingenious and amusing feat, but the same principle is involved in it as in the " plumbeous porridge " of Engineering. The heavier bodies are only sustained-by the momentum of the more rapidly flowing light fluid. Again what is the " tendency to continue its onward motion " which Engineenng says is sufficient to overcome gravity but an impulse received from the water. But admitting for the sake of argument that it has such a tendency in and of itself (its inertia perhaps is meant), the direction of such a force would be in a line parallel to the bottom. On what new principle of physics is it asserted that a force acting at nearly a right angle to the force of gravity will counteract gravity ? A proposition at once so entirely void of any foundation in the laws of force and motion, and so feebly sustained by argument will surprise the readers and admirers of our esteemed and usually accurate cotemporary. Does it not also tacitly admit its error when it says that " anything, however heavy, will swim in water if it only runs sufficiently fast. Is this not equivalent to saying the heavier the body the greater the velocity in the stream needed, not only to start it, but to Imp it up after it starts ? And what ground is there for asserting that such a body would sink " at once " should its velocity ever become less than that of the water? Let Engineering tie a cast-iron plate to a string and then throw it upon a very rapidly flowing stream, holding on tight to the string, and report the result. The experiment will be nothing more than flying a water kite.
This article was originally published with the title "What Keeps Solid Bodies Heavier than Water Suspended in a Running Stream?"