You see this in the different lengths of the diagonals in Fig. 144 and Fig. 146. The more nearly, therefore, the wind coincides with the current the more rapidly will a vessel be carried along before the wind. When, on the other hand, the angle at which two forces act upon a body is much greater than a right angle, they will propel it but a small distance. Thus if two forces act on a body in the directions D A and D C, Fig. 147, they will move it only the distance represented by the diagonal D B. This diagram represents the motion of a vessel sailing almost directly against a current by a wind the force of which is equal to that of the current, while Fig. 146 represents the motion of a vessel where wind and current being of equal force very nearly coincide. In the above diagrams I have supposed the forces to be equal; but the same truth can be shown in regard to unequal forces as seen in Fig. 143. 213. =Curved Motion.=--No single impulse can produce a curved motion. Neither can two or more impulses communicated at one time. In both of these cases the motion would be in a straight line. Curved motion may be produced by two forces, one of which gives it a single impulse, and the other acts upon it continuously. A familiar example you have in a ball whirled around at the end of a string. You can give it an impulse, and then, holding it in your hand, let it whirl. Here the impulse you give the ball is one force, and the tension of the string is the other, the latter acting continuously. Your hand holding the end of the string is the centre about which the motion revolves; the impulse which you have given the ball tends to make it fly away from the centre in a straight line, and hence is called the _centrifugal_ force; the tension of the string keeps it from thus flying off, and so is called the _centripetal_ force. When the earth, at the creation, was put in motion it would have moved in a perfectly straight line, were it not constantly drawn toward the sun by attraction, the continuous action of this latter force being the same as the tension of the string in the case of the whirling ball. The force of attraction, then, is the centripetal force of the earth, and the impulse which was given to it by the Creator in the beginning is its centrifugal force; and, balanced between these two forces, the earth and all the heavenly bodies move uniformly onward in their orbits. The centrifugal force you see in these illustrations is simply the tendency of motion to a straight line from the inertia of matter; and this is constantly counteracted by the centripetal force. [Illustration: Fig. 148.]