The Spanish Dancer.
The laws which govern the motion of bodies are capable of many pleasing illustrations, and the example which we now give of causing rotary motion is very interesting and easily performed.
Take a piece of card, and cut out a little figure, and paste or gum it in an erect position on the inside of a watch-glass. Then procure a black japanned waiter, or a clean plate will do, and, holding it in an inclined position, place the figure and watch-glass on it, and they will, of course, slide down. Next let fall a drop of water on the waiter, place the watch-glass on it, and again incline the waiter, and instead of the watch-glass sliding down, it will begin to revolve. It will continue to revolve with increasing velocity, obeying the inclination and position of the plane, as directed by the hand of the experimentalist. The reason of this is, in the first place, in consequence of the cohesion of the water to the two surfaces, a new force is introduced, by which an unequal degree of resistance is imparted to different parts of the watch-glass in contact with the waiter, and, consequently, in its effort to slide down, it revolves. Again, if the drop of water be observed, it will be seen that it undergoes a change of figure; a film of water, by capillary action, is drawn to the foremost portion of the glass, while, by the centrifugal force, a body of water is thrown under the under part of it. The effect of both these actions is to accelerate the motion, or, in other words, to gradually increase the speed.