The cause in both cases is the same. Here is another “law of nature,” so-called. Though we can neither explain nor understand why and how it is so, we see it to be one of the fixed rules of God’s working in every-day life throughout his universe. The law, as we see it, seems to be this: Everything which is at rest must remain at rest, until set moving by some cause outside of, or independent of itself; and everything which is once set moving, must continue moving in a straight line until checked. According to this a cannon-ball lying on the ground ought to remain there until it is set in motion; and, once set in motion by being fired from a cannon, it ought to go on forever. Exactly so--if nothing stops it. But the earth’s attraction draws the cannon-ball downward, and every time it strikes the ground it is partly checked. Also each particle of air that touches it helps to bring it to rest. If there were no earth and no air in the question, the cannon-ball might rush on in space for thousands of years. Why did the water get spilled? Because it necessarily continued moving in a straight line. Your sudden change of direction compelled the solid glass to make the same change, but the liquid water was free to go straight on in its former course; so it obeyed this law, and _did_ go on. Why did the ball pull hard at the string as you swung it round? Because at each instant it was striving to obey this same law, and to rush onward in a straight line. The pull of the string was every moment fighting against that inclination, and forcing the ball to move in a circle. Just such is the earth’s movement in her yearly journey round the sun. The string holding in the ball pictures the sun’s attraction holding in the earth. The pulling of the ball outward in order to continue its course in a straight line, pictures the pulling of our earth each moment to break loose from the sun’s attraction and to flee away into distant space. For the earth is not at rest. Each tick of the clock she has sped onward over more than eighteen miles of her pathway through the sky. Every instant the sun is dragging, with the tremendous force of his attraction, to make her fall nearer to him. Every instant the earth is dragging with the tremendous force of her rapid rush, to get away from him. These two pullings so far balance each other, or, more strictly, so far combine together, that between the two she journeys steadily round and round in her nearly circular orbit.