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which the attraction of a planet may be measured. One is by its attraction on the planets next to it, causing them to deviate from the orbits in which they would move if left to themselves. By measuring the deviations, we can determine the amount of the pull, and hence the mass of the planet.

The reader will readily understand that the mathematical processes necessary to get a result in this way must be very delicate and complicated. A much simpler method can be used in the case of those planets which have satellites revolving round them, because the attraction of the planet can be determined by the motions of the satellite. The first law of motion teaches us that a body in motion, if acted on by no force, will move in a straight line. Hence, if we see a body moving in a curve, we know that it is acted on by a force in the direction toward which the motion curves. A familiar example is that of a stone thrown from the hand. If the stone were not attracted by the earth it would go on forever in the line of throw, and leave the earth entirely. But under the attraction of the earth it is drawn down and down, as it travels onward, until finally it reaches the ground. The faster the stone is thrown, of course, the farther it will go, and the greater will be the sweep of the curve of its path. If it were a cannon ball, the first part of the curve would be nearly a right line. If we could fire a cannon ball horizontally from the top of a high mountain with a velocity of five miles a second, and if it were not resisted by the air, the curvature of the path would be equal to that of the surface of our earth, and so the