Buoys painted with red and black horizontal lines mark obstructions with channels on both sides. Buoys with white and black perpendicular stripes sometimes mark the middle of a channel and a ship should pass close to them. Buoys marking quarantine are yellow, while buoys marking the limits of anchorages are usually white. [Illustration: AUTOMATIC BUOYS _The whistle buoy at the left utilizes the motion of the waves to blow a whistle. The light buoy in the centre has an automatic light that burns gas stored in the body of the buoy. The bell buoy at the right carries a bell, against which four clappers are pounded by the action of the waves._ ] The whistling buoys and lighted buoys are, perhaps, the most interesting of the lot. Imagine a huge steel top, with a whistle placed at its point, and a large steel tube running through it from top to bottom, extending more than the height of the top above it. Imagine this top ten or twelve feet in diameter, and, with the tube, forty feet in height. Imagine this, then, floating in the water, point up, and with the tube below the surface. The end of the tube below the water is open. The end on which the whistle is mounted contains two openings. In one of these the whistle is placed. The other opening is closed by a valve which permits air to enter, but closes when the air tries to escape. This buoy is anchored in the water, and as the waves toss it up and down they rise and fall in the lower part of the tube. As they rise the air inside is compressed and is blown through the whistle causing it to sound. As the water in the tube falls, air is drawn through the valve, and again the waves force it through the whistle. This ponderous but simple “whistling” buoy requires no supplies and almost no attention. Periodically it is visited by a tender and is temporarily relieved of work while it is taken to the repair shop to be examined, repaired, and painted. Aside from that it needs no attention, yet constantly it moans as the waves sweep under it, and the greater the waves the greater is the volume of its sound. Bell buoys are equally simple and effective. These buoys are surmounted by a framework of steel from which a large bell is rigidly suspended. Several “clappers” are hinged about it so that, no matter how a wave may move the buoy, a clapper strikes the bell.