If a ball be allowed to drop from the mast-head of a ship _at rest_, it will strike the deck at the foot of the mast. If the same experiment be tried with a ship _in motion_, the same result will be observed. Because, in the latter case, the ball is acted upon simultaneously by two forces at right angles to each other--one, the momentum given to it by the moving ship in the direction of its own motion, and the other the force of gravity, the direction of which is square to that of the momentum. The ball being acted upon by the two forces together will not go in the direction of either, but will take a diagonal course, as shown in the following diagram, Figure 22. [Illustration: FIG. 22.] [Illustration: FIG. 23.] The ball passing from A to C by the force of gravity, and having at the moment of its liberation received a momentum from the ship in the direction A B, will by the conjoint action of the two forces, take the direction A D, falling at D, just as it would have fallen at C had the vessel remained at rest. In this way, it is contended by those who hold that the Earth is a moving sphere, a ball allowed to fall from the mouth of a deep mine reaches the bottom in an apparently vertical direction, the same as it would if the Earth were motionless. So far, there need be no discussion--the explanation is granted. But now let the experiment be modified in the following way:--Let the ball be thrown _upwards from_ the mast-head of a moving vessel; it will partake as before of two motions, the upward and the horizontal, and will take a diagonal course upwards and with the vessel until the two forces expend themselves, when it will begin to fall by the force of gravity only, and drop into the water far behind the ship, which is still moving horizontally. Diagram Figure 23 will illustrate this effect. The ball being thrown upwards in the direction A C, and the vessel moving from A to B, will cause it to pass in the direction A D, arriving at D when the vessel reaches B; the two forces having expended themselves when the ball arrives at D, it will begin to descend by the force of gravity in the direction D B H, but during its fall the vessel will have reached the position S, so that the ball will drop far behind it at the point H. To bring the ball from D to S _two forces_ would be required, as D H and D W; but as D W does not exist, the force of gravity operates _alone_, and the ball necessarily falls behind the vessel at a distance proportionate to the altitude attained at D, and the time occupied in falling from D to H.