time to rise on the following morning. Though this idea may appear very crude, it is far in advance of a theory held long after by the Greeks. The earth was supposed to be a plane surface surrounded by a river, and in it the sun had a bath every evening, rising "like a giant refreshed" on the following morning. Probably the sinking and subsequent rising of a diver suggested that the sun went through a similar performance. This imaginative idea gave way to the suppositions that the sun passed through a tunnel under the earth, or between roots or pillars, or behind a hill, his temporary absence leaving the earth in the darkness of night.

But the sun is not the only celestial body apparently in constant motion. We have seen in the first chapter that the whole heavens appear to perform a complete rotation from east to west every day. Even after man had discovered that the earth was a sphere, and that the heavens did not repose upon it, but surrounded it on all sides, it was held that the stars existed on a solid firmament which revolved round the earth. Our little globe was then considered to be immovable in the centre of the universe, and the decline of this idea. may be said only to have begun with Copernicus, about three centuries ago. This distinguished man demonstrated that the earth is constantly spinning round and round and that the heavens are at rest. The firmament thus appears to move from east to west in a day because the earth rotates from west to east in the same time. Every boy who has been on a merrygo-round at a fair knows that all the objects outside. appear to rush past him in a direction opposite to that in which he pursues his mad career. This

apparent motion is a reflection of the boy's real motion, and, by applying similar reasoning to the earth and the heavens, it will easily be understood that, so far as appearances are concerned, it does not matter which is considered to be in rotation. If the merry-go-round were driven so smoothly and uniformly that it was impossible to feel whether it was moving or not, the actual condition of the earth would be realised.

It is not to be wondered at that the theory of the earth's rotation was hotly disputed at the time of Copernicus. Until the fact that the earth and its atmosphere move with absolute uniformity is fully grasped, it is difficult for us to conceive that objects on the earth's equator are being whirled round with a velocity of one thousand miles an hour. At all events, it is much easier to think that the earth is at rest, and the celestial vault is moving round it, than to believe that the reverse is the case. After Copernicus, facts were produced which added to the probability of the earth's rotation. By watching the movements of spots upon the sun, our luminary was shown to have a spinning motion. What was more natural than to argue from analogy that the earth behaves in a similar fashion. At the present time, however, we have neither to rely upon probability nor analogy. Through the ingenuity of a famous French physicist named Foucault, the rotation of the earth can be proved in the following conclusive manner.

When an inanimate object, such as a stone, is set in motion, it goes on moving in the direction in which it was started, unless some disturbing influence deflects it from this path. As an example of this law of motion,

direction of motion in the case of a simpl can be illustrated by means of an experime

The feet of a gallows-shaped erection a circular piece of wood, which turns round

FIG. 14.-Model to show that the thread or fine wire suspending to and fro can be twisted without changing the direction

its centre.

From the middle of the crossgallows hangs a fine wire or thread, at which is a lead weight. Suppose the weig to one side and let go, so that it vib direction of the line joining the two upr when this has been done, let the disc be slowly turned round. The pendulum wi stubbornly to refuse to partake of this moti keep moving to and fro in the original dire no longer between the uprights. When have been rotated through one-quarter of

D

connecting them.

turn, the pendulum motion will be across the line Rotate the uprights half-way round, and the pendulum will again beat between them, though they will have changed sides. At three-quarters of a complete turn the line of movement again cuts straight across that which joins the uprights, and when the circular slab has been rotated through a whole circumference, the conditions are the same as at the beginning of the experiment. It is obvious, then, that a pendulum cannot easily have its

FIG. 15.-The left-hand figure shows a Foucault's pendulum started over a central line on a table in the northern hemisphere; the right-hand figure shows the apparent change after about two hours.

stand Foucault's experiment for proving of the earth (Fig. 15). In 1851 suspended a heavy iron ball from the Paris Pantheon by means of a fine wi hundred feet long. The pendulum th was then pulled out of its position of with a piece of thin string to a fixed string was afterwards burnt, and the pe its oscillations in a certain direction.

it was found that the direction of motion to the floor of the Pantheon, had alt appearances it seemed as if the plane pendulum vibrated were slowly moving same direction as that of the hands of on the floor. But our experiment h that a heavy moving body cannot easi out of its original path. For this r concluded that the whole of the buildin ally rotating in the opposite direction watch-hands, while the pendulum-bob 1 swinging in the way it was made to go. ment has been repeated many times si day, and a similar result has always b The earth's rotation is thus placed beyo bility of a doubt.

If a "Foucault's pendulum" could from a large gallows at the poles, a