Page images
PDF
EPUB

Using the new value, Newton again took up the calculation of the moon's deflection from a straight path, on the assumption of the force controlling it being the same as that which causes bodies to fall on the earth, but diminished in strength on account of the distance of the moon from the earth. The result now obtained showed that the central force should cause the moon to swerve from a straight line path by 53 thousandths of an inch in a second, which is exactly the amount that will produce the curved path described by the moon around the earth. As the calculations which were thus to extend the domain of gravity from the earth to the universe were drawing to a close, it is related that Newton was "so much agitated that he was obliged to desire a friend to finish them."

It must not be supposed for a moment that the problem of testing the theory of gravitation by the motion was so simple as it is here presented. In order to establish the principle, Newton had to prove that as regards gravitational attraction, a globe like the sun, moon, or earth behaves as if the force due to the whole of the mass resided at the centre alone. This conclusion was only arrived at after infinite labour and by the employment of a new mathematical method invented by him. By this same method also he was able to prove that the path of a body under the influence of attraction depending upon a central force could be an ellipse or any other related curve. The elliptical paths of the moon around the earth, the earth and other planets around the sun, and of all satellites around their primary planets were explained by this discovery; yet, as we have seen, Newton was content with having found it, and only through the intervention of Halley was the world made aware of it.

This is not the place to show that the law of universal gravitation provides a complete explanation of Kepler's three laws of planetary motion, or to discuss its profound significance. A law of Nature is tested by its ability to meet all the cases to which it may be applied; and in not a single instance has the law of gravitation been found wanting in this respect. It explains the fall of bodies on the earth, and the motions of the planets and their satellites; it enables the paths of comets to be calculated, and the disturbances to which they may be subjected in passing near more massive bodies; it is used to determine the masses of bodies in our solar system, and of stars revolving round one another, and is applied to calculate the tide-raising effects of the sun and moon; while by it tables of the moon's movements are calculated to a high degree of perfection for use in determining longitude at sea; and it does all this without any need of amendment.

It has been truly said that "The tendency of the human mind is to exaggerate the possibilities of the unknown." Wherever there is ignorance, Nature is dreaded as much as a child dreads darkness. The appearance of a comet in the sky caused whole nations in earlier days to tremble with fear because nothing was known of the nature and movements of these bodies. From the time that Newton showed that comets travel round the sun in 'definite paths under the control of gravitational attraction, the feeling of awe and anxiety formerly produced by such celestial visitors has been diminishing. They are now looked upon as interesting spectacles instead of being regarded as heralds of disaster.

Edmund Halley was inspired by Newton's work to calculate the paths of a number of comets. He found

that three comets, which appeared in 1531, 1607, and 1682, had practically the same path or orbit, and he concluded that they were really one and the same body travelling around the sun in a period of about seventyfive years. He predicted, therefore, that the comet would appear again in the year 1758, or thereabouts; that is, seventy-six years after 1682. He knew he would not be alive to see if the prediction was fulfilled, but he expressed the hope that, when the comet was seen, posterity would remember the prediction had been made by an Englishman. This was the first prediction of the periodic return of a comet, and Halley's boldness in making it was justified completely. The comet, which was anxiously awaited, appeared in 1758 and again in 1835 and 1910. By examining old records, Halley's comet, as it is properly named, has been traced back in steps of about seventy-five years to the year 240 B.C.

Newton showed that the law of gravitation was sufficient to account for the motion of the comet of 1680 to which he applied it; and since then the paths of hundreds of comets have been calculated on the same principle. Most comets are unannounced visitors to the solar system, called from the depths of space by the attractive influence of the sun, but following paths which carry them away again into the outer darkness. Some, however, like Halley's comet, tråverse orbits which are closed curves, and these return after an interval which may be reckoned in years or hundreds of years. But the path of every comet of which sufficient observations are available is calculated upon the basis of the law of gravitation. The apparently adventitious motions of these bodies are thus reducible to perfect law and order; and the decline of the superstitious

dread in which they had been held may be said to have begun with Newton's discovery.

A notable example of the prediction of the existence of a body from consideration of its gravitational influence was afforded by the discovery of the planet Neptune. After the planet Uranus had been discovered in 1781 by Sir William Herschel, astronomers calculated the path in which it moved, and predicted the positions it should occupy from time to time. The planet was found, however, to be slightly behind or ahead of its calculated position. If we know a train is moving at a certain rate, we can say where it ought to be on the line at a certain time, and if it is not there we assume that something has happened. So it was with the planet Uranus: the difference between the actual and the calculated positions of the planet was assumed to be due to the disturbing influence of some unknown body beyond it.

There was every confidence that a massive globe somewhere in the darkness of space was making its presence manifest; and though the problem of finding the place of the disturber was very difficult, it was solved by two mathematicians—one an Englishman named Adams and the other a Frenchman named Le Verrier. In September, 1846, Sir John Herschel, son of Sir William Herschel, said: "We see it [the probable new planet] as Columbus saw America from the shores of Spain. Its movements have been felt trembling along the farreaching line of our analysis with a certainty hardly inferior to that of ocular demonstration." On the twenty-third of the same month a German astronomer, Dr. Galle, found the new body close to the place which calculation had shown it ought to occupy. This is probably the greatest triumph of mathematics applied to the law of gravitation yet achieved.

The law of gravitation has proved to be a universal key which has opened the door of many secret places in Nature, yet what the key itself is—or wherein lies the cause of the attraction of gravitation-is still to seek. Now that the interstellar ether is becoming almost as familiar to us as a household word, and its recondite properties are being unravelled, there is, perhaps, some prospect of the problem being solved. Efforts have been made to discover whether gravitational force is propagated instantaneously through space. If we could suddenly create or destroy a centre of attraction, as we can a beam of light, the answer would be fairly easy. But as it is, we can only watch whether changes of position among the heavenly bodies produce their proper effect upon other bodies at once or after a measurable lapse of time. So far, no evidence of such a lapse of time has been forthcoming, although the most favoured theories demand it.

One of those theories, originally due to Le Sage, supposes that an infinite number of very small "ultramundane" particles is constantly traversing space in all directions. They are partly intercepted by ponderable matter. Hence two bodies will shield each other from the particles on one side, and the bombardment on the outside will drive them together, producing an apparent attraction" between them. Reasons have been found for the conclusion that at a certain great distance gravitational attraction is intercepted altogether. If this were true, Newton's grand conception of universal gravitation would have to be modified, and we should have to consider the visible universe as held together by beams of light.

66

Next to the law of gravitation, the greatest generalisation established by science, and the one to which likewise

« PreviousContinue »