Every one knows that a top will stand upright only when it is spinning. Most tops when spinning will stand very rough treatment without being upset. The whip-top will stand a severe lashing. Spin a top upright and give it a knock. It will go round in a circle in a slanting position, and after a time will right itself. If the top is struck toward the south it will not bow toward the south but toward the east or west. In throwing a quoit, the quoit must be given a spinning motion or the thrower cannot be certain how it will alight. A coin thrown up with a spinning motion will not turn over. The quoit and the coin are like the top. They will not turn over easily when spinning. For the same reason a rifle bullet is set spinning by the spiral grooves in the bore of the gun, and it goes straight to its mark. With a smooth-bore gun that does not set the bullet spinning the gunner cannot be sure of his aim. It took a long time to discover that the spinning top is a useful machine. It is useful because of its steady motion, because it is difficult to turn over. It was discovered by Newton long ago that every moving object tries to keep on in the direction in which it is moving. A moving object always requires some force to change its direction. The spinning top is a beautiful illustration of this principle. The top that is most useful is the gyroscope top (Fig. 103). It is mounted on pivots so arranged that the top can turn in any direction within the frame that supports it. If the top is set spinning one may turn the frame in any direction, but the top does not change direction. The axis of the top will point in the same direction all the while the top is spinning, no matter how the supporting frame is moved about. The top will spin on a string. If attached inside a box the box can be made to stand on one corner while the top is spinning. [Illustration: FIG. 103--A TOP THAT SPINS ON A STRING] This top, which is so hard to upset, has been used in ships to prevent the ship being rolled by the waves. A large fly-wheel is mounted in the middle of the vessel on a horizontal axle. A fly-wheel is only a large top. It spins with a steady motion, and because of its larger size it is very much harder to overturn than a toy top. The fly-wheel in the ship resists the rolling force of the waves and steadies the ship, so that even with high waves the rolling can scarcely be felt. The waves do not so readily break over the ship when thus steadied by the revolving wheel.