Fig. 102.
Velocities of streams.—The velocity of streams varies greatly. The slower flow of rivers has a velocity of less than three feet per second, and the more rapid, as much as six feet per second, which gives respectively about two and four miles per hour. The velocities vary in different parts of the same transverse section of a stream, for the air upon the surface of the water, as well also as the solid bottom of the stream, has a certain effect in retarding the current. The velocity is found to be greatest in the middle, where the water is deepest, Fig. 102, somewhere in m, below the surface; then it decreases with the depth towards the sides, being least at a and b.
Fig. 103.
Appearance of the surface during a discharge.—A vessel containing a liquid, discharging itself through an orifice, does not always preserve a horizontal surface. When the vein issues from an orifice in the bottom of a vessel, and the level of the liquid is near the orifice, the liquid forms a whirlpool, Fig. 103. If the liquid has a rotary movement, the funnel is formed sooner; if the orifice is at the side of the vessel, there is a depression of the surface upon that side, above the orifice, Fig. 104. These movements depend upon the form of the vessel, the height of the liquid in it, and the dimensions and form of the orifice.
Fig. 104.
In order to verify many of the laws of hydraulics in an accurate manner, it is necessary to maintain a uniform pressure on the escaping liquid, thereby obtaining a constant velocity at the orifice. This may be done in various ways, as by allowing the water to flow into the vessel in a little larger quantity than can escape from the orifice, the excess being discharged over the upper edge of the vessel; also by means of the syphon.
By suspending solid particles, such as charred paper, pulverized in the water, we render the currents that are formed visible. These solid particles arrange themselves, in curved lines, towards and into the orifice, as a center of attraction, [Fig. 105]. The particles in immediate contact with the orifice, not moving so easily as those within, must cause contraction; so, also, we can see that gravity in accelerating the velocity, must cause continual decrease in the section of the jet.