195. =Relation of Shape to Velocity.=--The resistance of air or water to a flat surface is greater than to a convex one, because the latter readily turns the particles to the one side and the other. So, also, a concave surface is resisted much more than a flat one, because the particles of the air or water can not so easily escape sideways. Fishes are of a spindle-like and slender shape, that they may have as little resistance as possible from the water. It is for this reason that a fish has no neck, for if it had one the upper portion of its body would, from the resistance of the water striking against it, prove a serious impediment to rapidity of motion. Mankind have in some measure imitated the shape of fishes in their boats and ships. Boats which are intended to bear light burdens and go swiftly are made very long and narrow. The webbed feet of water-fowls, when they are moved forward, are folded up so as to meet with as little resistance as possible; but when they are moved backward they are spread out so as to press against the water a broad concave surface. For the same reason the wings of a bird are made convex upward and concave downward; and when it moves its wing upward it makes it cut the air somewhat edgewise, but in moving it downward it presses directly with the whole concave surface. 196. =Friction.=--Friction is generally an obstacle to motion. When we roll a ball, the more rough is the surface on which we roll it the greater is the friction and the sooner is the ball stopped. Friction lessens the rapidity of motion in machinery, and to prevent this as far as possible oiling and other expedients are employed. But sometimes friction is a cause of motion, as, for example, the friction of the driving-wheels of a locomotive upon the rails. In this case the wheel pushes backward on the rail at each successive point of contact. To make this clear, suppose a common wheel is deprived of its rim and is made to revolve on the ends of its spokes. The end of each spoke gives a backward push as it strikes the ground. Now the rim of a wheel makes the same pushes, but they are more numerous--they are continuous, being made by all the successive points in the rim. Sometimes the rails of a railroad are too smooth from frost or some other cause, and then sand is thrown upon them to give the locomotive a start. The sand serves to prevent the wheels from sliding by enabling them to get some hold upon the rails in their backward pushes.