This inability of ours to give a good physical explanation of gravitation has led numerous paradoxers to doubt or even deny that there is any such thing. But fortunately we have a simple laboratory experiment that helps us. Unexplained it may ever remain, but that there can be attraction between physical objects connected by no visible link is proved by the behavior of an ordinary magnet. Place a small piece of steel or iron near a magnetized bar, and it will at once be so strongly attracted that it will actually fly to the magnet. Any one who has seen this simple experiment can never again deny the possibility at least of the law of attraction as stated by Newton. Its possibility once admitted, the fact that it can predict the motions of all the planets, even shown to the minutest details, transforms the possibility of its birth into a certainty as strong as any human certainty can ever be. But this demonstration of Newton’s law is limited strictly to the solar system itself. We may indeed reason by analogy, and take for granted that a law which holds within our immediate neighborhood is extremely likely to be true also of the entire visible universe. But men of science are loath to reason thus; and hence the fascination of researches in cosmic astronomy. Analogy points out the path. The astronomer is not slow to follow; but he seeks ever to establish upon incontrovertible evidence those truths which at first only his daring imagination had led him to half suspect. If we are to extend the law of gravitation to the utmost, we must be careful to consider the law itself in its most complete form. A heavenly body like the sun is often said to govern the motions of its family of planets; but such a statement is not strictly accurate. The governing body is no despot; ’tis an abject slave of law and order, as much as the tiniest of attendant planets. The action of gravitation is mutual, and no cosmic body can attract another without being itself in turn subject to that other’s gravitational action. If there were in our solar system but two bodies, sun and planet, we should find each one pursuing a path in space under the influence of the other’s attraction. These two paths or orbits would be oval, and if the sun and planet were equally massive, the orbits would be exactly alike, both in shape and size. But if the sun were far larger than the planet, the orbits would still be similar in form, but the one traversed by the larger body would be small. For it is not reasonable to expect a little planet to keep the big sun moving with a velocity as great as that derived by itself from the attraction of the larger orb. Whenever the preponderance of the larger body is extremely great, its orbit will be correspondingly insignificant in size. This is in fact the case with our own sun. So massive is it in comparison with the planets, that the orbit is too small to reveal its actual existence without the aid of our most refined instruments.