Page:Elementary Text-book of Physics (Anthony, 1897).djvu/157

§ 123] of the circle, in the direction of the motion of the fluid rotating on the inner surface of the ring.

(8) The fluid encircled by the ring moves along its axis in the direction of the motion of the ring, and with a greater velocity.

(9) If two circular vortices move along the same axis, one following the other, the one in the rear moves faster, and diminishes in diameter; the one in advance moves slower, and increases in diameter. If the strength and size of the two be nearly equal, the one in the rear overtakes the other, and passes through it. The two now having changed places, the action is repeated indefinitely.

(10) If two circular vortices of equal strength move along the same axis toward one another, the velocities of both gradually decrease and their diameters increase. The same result follows if one such vortex move toward a solid barrier.

The preceding statements apply only to vortices set up in a perfect fluid. They may, however, be illustrated by experiment. To produce circular vortices in the air, we use a box which has one of its ends flexible. A circular opening is cut in the opposite end. The box is filled with smoke or with finely divided sal-ammoniac, resulting from the combination of the vapors of ammonia and hydrochloric acid. On striking the flexible end of the box smoke-rings are at once sent out.

The smoke-ring is easily seen to be made up of particles revolving about a central core in the form of a ring. With such rings many of the preceding statements may be verified.

An illustration of the open vortex is seen when an oar-blade is drawn through the water. By making such open vortices, using a circular disk, many of the observations with the smoke-rings may be repeated in another form.

123. Air-pumps.—The fact that gases, unlike liquids, are easily compressed, and obey Boyle's law under ordinary conditions of temperature and pressure, underlies the construction and operation of several pieces of apparatus employed in physical investigations. The most important of these is the air-pump.