THE STORY OF HALLEY'S COMET. The familiar name of "Halley's Comet" has an abiding interest for all who care for the story the heavens are telling. It is closely associated with events which have contributed largely towards moulding the destiny of Europe. And it is not too much to say that students of astronomy in every land honor the memory of our distinguished countryman, whose labors in an entirely new field of research enabled him to foretell, that the celestial wanderer that appeared in our skies in 1682 would come again after the lapse of three-quarters of a century. Since then it has paid us two more visits, and we are now looking forward to a third. The light which science was then, for the first time in history, shedding upon the structure of the heavens inspired both Newton and Halley with the idea that the comets were perhaps controlled in their movements by the same influence as that which Newton already believed and was just on the point of demonstrating, held the Moon in its orbit. The happy thought had occurred to them while gazing upon the truly imposing comet which astonished the world in the winter evenings of 1680. It came from regions high above the plane of the ecliptic, plunging down towards the Sun with amazing velocity. For a short time it was lost sight of; then, emerging from the Sun's rays, almost grazing his sides as it passed round him, it put forth a train-straight, fierce, long-extending to a distance of many millions of miles. With like marvellous speed it ascended into the heavens above and swept onward into the abyss of space, followed with the keenest curiosity by every possessor of a telescope. Newton, using his seven-foot reflector, was the last to bid the stranger a final adieu, on the 19th of March 1681.' With remarkable insight Newton grasped the fact that comets were analogous to the planets in constitution and movement, and that the tail was formed of a very fine, slendor vapor raised by the Sun's heat while the body was in his immediate vicinity. Reasoning thus, he applied his method of geometrical construction which had revealed to him the law of gravitation. He found that a curve could be formed which would include all the places, he assumed, in which the comet would move. The problem was difficult but he solved it, and proved conclusively that the comet pursued an extremely eccentric course embraced within an oval which he thought would occupy it about 575 years to complete. But the fuller and more complete investigations of recent years render it not improbable that its mighty rush round the Sun may have been sufficient to carry it off for ever, and that it may now be winging its lofty flight towards some other system of worlds, in which it may possibly excite amazement or alarm, as here. From the general application of the law it follows that comets move in one or other of the conic sections-either the circle, ellipse, parabola, or hyperbola-having one of their foci in or near the centre of the Sun, and by radii drawn from the Sun describe areas proportionable to the times. Having discovered the reason of the great law of the celestial movements in the single principle of mutual attraction of one mass of matter towards an 1 Halley first caught sight of the comet while on his way to Paris. There he met with Cassini, and together they made careful observations of Its exact distance from the nearest fixed star, and night by night noted its changed positions. These observations were of great service to Newton; they enabled him taken with his own measurements, to fix the comet's orbit. other. Newton gave it a mechanical interpretation, embracing the comets, and indeed, all heavenly bodies. The first book of his Principia, showing that this was a necessary consequence flowing from the principle, was presented to the Royal Society in April 1686. He there lays the foundation of cometary astronomy, and he it was who made Halley's prediction possible. And yet, but for Halley, this great work might never have been printed. Halley, in 1684, paid a visit to Newton at Cambridge, and learned the good news that he had brought his demonstrations to perfection. Newton could not afford the cost of printing and publishing his work, but Halley, though not a wealthy man, undertook to bear the expense, and eagerly gave up his time to correcting the proof and in every way pressing forward to the utmost the printing and issue from the press. From this moment a new era dawned upon the world of science; men awakened to a great truth, and began to understand something of the bond which holds in perfect harmony the stars in their courses in due relationship one to another; everywhere was manifest law, immutable, irresistible. On the 15th of August 1682 the body whose career we are tracing came into view. It was first detected by Flamsteed's assistant at the Greenwich Observatory, while scanning the northern heavens with a telescope. Flamsteed, the first Astronomer Royal, and Halley, his successor, kept a close watch upon the comet and followed its course over the sky. It was noticed that it had a somewhat oval-shaped body, and that it was moving in a path contrary to the regular course of the planets, or "retrograde," and but little inclined to the ecliptic. By the 26th the head, though no larger than a star' of the second magnitude, put forth a tail of about 12° long, which waved towards the eastern side. Astronomers at Paris now caught sight of it, and both Picard and La Hire thought they saw phases in the nucleus similar to those of the Moon. If this were really so, it would imply that the comet was solid and compact, as Newton from his observations of the one of 1680 believed the comets to be. But the immense nebulosity which enshrouds the head renders the inference very doubtful. The tail lengthened considerably as the comet came nearer to the Sun; and now was remarked by all observers a sparkling or quivering movement running through its entire length. A little later a jet of luminous matter shot out towards the Sun, then, apparently meeting with some resistance, it fell backwards on all sides and mingled with the tail. Hevelius, at Dantzic, thought the phenomenon so peculiar that he represented it in a drawing. We now know that this kind of eruption, so suggestive of internal agitation, is common to almost all comets when in the neighborhood of the Sun. Indeed, as regards size, form, or any of the marvellous characteristics which mark the flight of comets through space, Halley's is of the average type. It suffices for us that it was the one which Halley fixed upon for his investigation into the mystery and movements of these hirsute visitants to our domain. Interest therefore centres in his labors, labors which he himself says were "prodigious"; and this may well be so, considering the state of analytical science at that time. Save Newton alone, there was no other man living who could have successfully dealt with the difficulties of the problem. It was Halley's good fortune to be closely associated in astronomical work with Sir Isaac Newton. He followed his advice implicitly, and adopted, on Newton's suggestion, Flamsteed's observations of the comet for the basis of his calculations. He assumed parabolic elements, though he thought it likely that it revolved in an orbit of extreme eccentricity, reaching far out into the heavens. And again acting on the advice of Newton, Halley made diligent search among ancient and modern historical records for the purpose of seeing whether mention was made of any comet the date of whose appearance, position, and movement agreed with the results his calculations had given. Casting his eyes over the table of cometary apparitions he had prepared for comparison, he was particularly struck with the similarity which existed between the comets of 1531, 1607, and 1682, in their having the like situation of their planes and perihelions. All the elements were so near alike that, after a reinvestigation, he felt little hesitation in concluding that they represented one and the same comet that had made three revolutions in its elliptical orbit. Halley in 1704 presented the results of his investigations to the Royal Society in a memoir entitled Astronomia Cometica Synopsis. It includes a table of twenty-four comets, beginning with the year 1337 and ending with the year 1698. He explains its use, thus: The principal use of the tables of the elements of their motion, and, indeed that which induced me to construct it, is that whenever a new comet shall appear we may be able to know by comparing together the elements whether it be any of those which had appeared before, and consequently to determine its period and the axis of its orbit, and to foretell its return. And, indeed, there are many things which make me believe that the comet which Apian observed in the year 1531 was the same with that which Kepler and Longomontanus more accurately observed in the year 1607, and which I myself have seen return in the year 1682. All the elements agree and nothing seems to contradict this my opinion, except that there is an inequality in the times of revolution, but this is not so great that it cannot be attributed to physical causes. For example, the motion of Saturn is so disturbed by the other planets, and especially by Jupiter that his periodic time is uncertain to the extent of several days. How much more liable to such perturbations is a comet which recedes to a distance nearly four times greater than Saturn and a slight increase in whose velocity would change its orbit from an ellipse to a parabola? The identity of these comets seems to be confirmed by the fact that in the summer of the year 1456 a comet was seen passing retrograde between the Earth and the Sun in nearly the same manner, and although it was not observed astronomically, yet from its period and path, I cannot think different from those I have just mentioned. Hence [he concludes] I may venture to foretell that it will return again in the year 1758. These explorations into the domain of the comet-world had a higher significance than the question of periodicity. The two illustrious astronomers had seen that these apparently erratic bodies are confined to no region of space; that they enter the planetary spaces at all angles with the plane of the ecliptic, coming no one could tell whence, or whither bound! nor at the outset could it be known whether the object whose characteristics declared it to be a comet would fall into the Sun, strike against one of the planets, or, indeed, collide with the Earth. It was recognized that inquiry into the nature and movement of Halley's comet had deprived these bodies for ever of their portentous character. But while relieving mankind from dread of cometary supernatural influence the new astronomy had brought to light a material danger, one which had never before been suspected. Here was work for the geometer of a kind to call forth his keenest scrutiny. Following up the inquiry, it was found that three or more exact observations, made on dif ferent dates, of the comet's position in relation to any fixed star near to which it might be situated, would show how far its course diverged from a straight line, and would thus afford the mathematician data from which he could construct a curve showing its exact path to and from the Sun. His ephemeris, indeed, may show whether the Earth, or any other of the planets, would be in its way, and tell the date and hour of its transit at every important point along its track. The chance of a collision, however, is so smallone in many millions-that the risk is hardly worth a moment's serious thought. Yet "the Chariot of Fire" which the year before careered through our neighborhood had left a vivid impression on the public mind. It had inspired Whiston with visions of dismay and destruction, and caused Halley to look closely into its movements. Halley's investigation led to the startling result that the comet had, on the 11th of November, when passing through the descending node, approached the Earth's path within a semi-diameter of the Earth. The discovery caused Halley to reflect upon the question as to what would have happened had the Earth and the comet arrived at the same time at the place where the two orbits intersect. suming the comet's mass to be comparable to that of the Earth, he concluded that their mutual gravitation would have caused a change in the position of the Earth's orbit and consequently in the length of the year. This train of thought led him on to consider what the result of an actual encounter would be, and he says, "If so large a body with so rapid a motion were to strike the Earth-a thing by no means impossible the shock might reduce this beautiful world to its original chaos." As A vivid light was thus cast upon a fascinating chapter in the history of our planet. To Dr. Whiston-he who succeeded Newton in the Lucasian chair of Mathematics at Cambridge-it opened a vista exhibiting the comet as the physical cause of the Deluge; and in the fulness of his prophetic vision he declared it to be the divinely appointed agent that would bring about the General Conflagration by involving the world in flames as it passed the Earth on its outward course from the Sun. Halley, too, though he could not join Whiston in his wonderful flight of fancy, was deeply impressed with the idea of a possible encounter between the Earth and a comet, and the consequences which must ensue therefrom. It seemed to him not improbable that the Earth had at some remote period been struck by a comet, which coming upon it obliquely had changed the position of its axis of rotation, the North Pole having originally, he thought, been at a point near to Hudson's Bay. To this cause he referred the rigor of the climate of North America; and the wide distribution of marine substances over the Earth's surface he considered lent support to his conjecture. Laplace a century later gave the sanction of his analytical reasoning to this view, and depicted in dismal colors the effect which would result to our globe were a comet at all comparable to the Earth in mass to strike against it. He says: The seas would abandon their ancient beds and rush towards the new equator, drowning in one universal deluge the greater part of the human race. We see, then, in effect why the ocean has receded from the high lands upon which we find incontestable marks of its sojourn; we see how the animals and plants of the south have been able to exist in the climate of the north where their remains and imprints have been discovered. Physicists of the present day, however, do not favor this view. Lord Kelvin and Sir George Darwin assure us that not only is the possible amount |