We are about three millions of miles nearer to the sun in the winter than we are in the summer; but from the more oblique or slanting direction of the rays of the sun during the winter season, we do not derive any increased heat from the greater proximity. The sun, therefore, apparently varies in size; but this seeming difference is so trifling that it is of no importance in the discussion: and here we may ask, why does [Page 24] the earth move round the sun? Because it is impelled by _two forces_, one of which has already been fully explained, and is called the _centrifugal_ power, and the other, although termed the _centripetal_ force, is only another name for the "attraction of gravitation." [Illustration: Fig. 33.] [Illustration: Fig. 34.] To show their mutual relations, let us suppose that, at the creation of the universe, the earth, marked A, was hurled from the hand of its Maker; according to the law of inertia, it would continue in a straight line, A C, for ever through space, provided it met with no resistance or obstruction. Let us now suppose the earth to have arrived at the point B, and to come within the sphere of the attraction of the sun S; [Page 25] here we have at once contending forces acting at right angles to each other; either the earth must continue in its original direction, A C, or fall gradually to the sun. But, mark the beauty and harmony of the arrangement: like a billiard-ball, struck with equal force at two points at right angles to each other, it takes the mean between the two, or what is termed the diagonal of the parallelogram (as shown in our drawing of a billiard-table), and passes in the direction of the curved line, B D; having reached D, it is again ready to fly off at a tangent; the centrifugal force would carry it to E, but again the gravitating force controls the centripetal, and the earth pursues its elliptical path, or orbit, till the Almighty Author who bade it move shall please to reverse the command. [Illustration: Fig. 35.] The mutual relations of the centripetal and centrifugal forces may be illustrated by suspending a tin cylindrical vessel by two strings, and having filled it with water, the vessel may be swung round without spilling a single drop; of course, the movement must be commenced carefully, by making it oscillate like a pendulum. [Illustration: Fig. 36.]