Next to the glory of having been the means of bringing about the publication of Newton's _Principia_, the greatest achievement of Halley, the second Astronomer Royal, was that he was the first to predict the return of a comet. Newton had shown that comets were no lawless wanderers, but were as obedient to gravitation as were the planets themselves, and he also showed how the orbit of a comet could be determined from observations on three different dates. Following these principles, Halley computed the orbits of no fewer than twenty-four comets, and found that three of them, visible at intervals of about seventy-five years, pursued practically the same path. He concluded, therefore, that these were really different appearances of the same object, and, searching old records, he found reason to believe that it had been observed frequently earlier still. It seems, in fact, to have been the comet which is recorded to have been seen in 1066 in England at the time of the Norman invasion; in A.D. 66, shortly before the commencement of that war which ended in the destruction of Jerusalem by Titus; and earlier still, so far back as B.C. 12. Halley, however, experienced a difficulty in his investigation. The period of the comet's revolution was not always the same. This, he concluded, must be due to the attraction of the planets near which the comet might chance to travel. In the summer of 1681 it had passed very close to Jupiter, for instance, and in consequence he expected that instead of returning in August 1757, seventy-five years after its last appearance, it would not return until the end of 1758 or the beginning of 1759. It has returned twice since Halley's day, a triumphant verification of the law of gravitation; and we are looking for it now for a third return some ten years hence, in 1910. Halley's comet, therefore, is an integral member of our solar system, as much so as the earth or Neptune, though it is utterly unlike them in appearance and constitution, and though its path is so utterly unlike theirs that it approaches the sun nearer than our earth, and recedes farther than Neptune. But there are other comets, which are not permanent members of our system, but only passing visitors. From the unfathomed depths of space they come, to those depths they go. They obey the law of gravitation so far as our sight can follow them, but what happens to them beyond? Do they come under some other law, or, perchance, in outermost space is there still a region reserved to primeval Chaos, the 'Anarch old,' where no law at all prevails? Gravitation is the bond of the solar system; is it also the bond of the Universe? CHAPTER IX THE MAGNETIC AND METEOROLOGICAL DEPARTMENTS Passing out of the south door of the new altazimuth building, we come to a white cruciform erection, constructed entirely of wood. This is the Magnet House or Magnetic Observatory, the home of a double Department, the Magnetic and Meteorological.