Fig, 150.
215. Bends in Rivers.—We see the operation of the centrifugal force in the bends of rivers. When a bend has once commenced in a river it is apt to increase, for as the water sweeps along the outer bank of the bend it presses strongly against it, just as the water in the whirled bucket, § 214, presses against its sides, by its centrifugal tendency, or, in other words, its tendency to assume a straight motion. Of course the result is a wearing away of this outer bank, and in proportion to the looseness of the material of which it is composed and the velocity of the river's current. And when one bend is formed another is apt to form below, but in an opposite direction. The water, by sweeping along the bend a, Fig. 149, is directed by it toward the opposite bank at b, and makes a bend there also. It is in this way that a river, running through a loose soil, the Mississippi, for example, acquires a very serpentine course. As the water in the whirled bucket rises around the sides, so in the river the water will be higher against the bank a than on the opposite side. Eddies and whirlpools are produced on the same principles, when water is obliged to turn quickly around some projecting point. If a current were moving swiftly along the shore a toward the point b, Fig. 150, it would be directed outward by the resistance of this projection, and so a depression would be left at c, just behind it, and this depression would be surrounded by a revolving edge of water.
Fig. 151.
216. Application of the Centrifugal Force in the Arts.—Much use is made of the centrifugal force in the arts, but I will give but two examples. In the art of pottery the clay is made to revolve on a whirling table, the workman at the same time giving the clay such shape as he chooses with his hands and various instruments. In doing this he constantly has reference to the centrifugal force, giving the table a velocity proportioned to the amount of this force which is needed in each stage of the operation. The most beautiful application of this force that I have ever witnessed is in the manufacture of common window-glass. The glass-blower gathers up on the end of his iron tube a quantity of the melted glass, and blows it out into a large globe. When it is of sufficient size and thinness he places it on a rest, as you see in Fig. 151 (p. 169). A second man now comes with a rod having some melted glass on the end, and attaches this to the globe at a point opposite to that where the tube of the first man is joined to it. There now comes a boy, and, giving this tube a quick blow, severs its connection with the globe, leaving a hole in the globe where the glass breaks out. The second man, having the globe attached to his rod, carries it to a blazing furnace, and resting the rod on a bar at its mouth, puts the globe directly into the flame. The glass is soon softened, and he whirls the globe continually around. The hole in the globe enlarges by the centrifugal force, and at length by this force the globe is changed into a flat, circular disk. Panes of glass which are called bull's-eyes are cut from the centres of these disks.
Fig. 152.
217. Steam-Governor.—The operation of the centrifugal force is beautifully exemplified in this regulator of the steam-engine. It consists of two heavy balls, Fig. 152, suspended by bars from a vertical axis, the bars being connected to the axis by hinges. The bars have also a hinged connection at their lower ends with two smaller bars, and these latter have a similar connection with a collar that slides up and down on the axis. Now the faster the axis turns the farther the balls fly out from it, from the centrifugal force, and the higher the collar slides up on the axis. From the collar extends, as you see, a lever. This is connected with a valve in the steam-pipe, and so regulates the amount of steam that enters the working part of the engine. The object of this ingenious contrivance is to make the engine regulate its own velocity. When it is not working too fast the valve in the steam-pipe is wide open. But the moment that it works too rapidly the balls extend out far from the axis, so that the collar rises, and by the lever partly closes the valve. Less steam, therefore, can come to the engine, and the engine working in consequence less rapidly, the balls fall again, opening the valve. You see, then, that the regulation of this valve by the governor effectually prevents the action of the engine from becoming too rapid.
