Even if the researches of Le Verrier and of Adams had never been undertaken it is certain that the distant Neptune must be some time discovered; yet that discovery might have been made in a manner which every true lover of science would now deplore. We hear constantly of the discovery of minor planets, yet no one attaches to such achievements a fraction of the consequence belonging to the discovery of Neptune. The danger to be feared by delay was, that Neptune should have been discovered by simple survey work, just as Uranus was discovered, or just as the hosts of minor planets are now found. In this case the science of Theoretical Astronomy, the great science founded by Newton, and raised to a marvellously interesting and beautiful system by the labours of Lagrange and Laplace, would have been deprived of its most brilliant illustration.

Neptune had, in fact, a very narrow escape on at least one previous occasion of being discovered in a very simple way. This was shown when sufficient observations had been obtained to enable the path of the planet to be calculated. It was then possible to trace back the movements of the planet among the stars and thus to institute a search in the catalogues of earlier astronomers to see whether they contained any record of Neptune. It was soon found that the place of the planet on May 10th, 1795, must have coincided with the place of a star recorded on that day in the "Histoire Céleste" of Lalande. It further appeared by actual examination of the heavens that there was no star in the place indicated by Lalande, so the fact that this was really an observation of Neptune was placed quite beyond doubt. When reference was made to the original observations of Lalande a matter of the very greatest interest was brought to light. It was there found that he had observed the same star (for so he regarded it) both on May 8th and on May 10th; on each day he had determined its position, and both are duly recorded. But when he came to prepare his catalogue and found that the places on the two occasions were different, he discarded the earlier result, and merely printed the later. Had Lalande but had the courage to believe implicitly in his own observations, an immortal discovery lay in his grasp; had he man

fully said, “I was right on the 10th of May and I was right on the 8th of May; I made no mistake on either occasion, and the object I saw on the 8th must have moved between that and the 10th," then he must without fail have found Neptune. But had he done so, how great would have been the loss to science! The discovery of Neptune would then merely have been an accidental reward to a laborious worker, instead of being one of the most glorious achievements in the loftiest department of human reason.

With the conclusion of the brief sketch here given of the discovery of Neptune, we nearly conclude all that can be narrated of this planet. If we fail to see in Uranus any of those features which make Mars or Venus, Jupiter or Saturn such attractive telescopic objects, what can we expect to find in Neptune, which is half as far again as Uranus? With a good telescope and a suitable magnifying power we can see that Neptune has a disc, but no features on that disc can be identified. We are consequently not in a position to ascertain the period in which Neptune rotates around its axis, though from the general analogy of the system we must feel assured that it really does rotate. More successful have been the attempts to measure the diameter of Neptune, which is found to be nearly 35,000 miles, or more than four times the diameter of the earth. It would also seem that, like Jupiter and like Saturn, the planet must be enveloped with a vast cloud laden atmosphere, for the density of the globe is only about one-fifth that of the earth. This great globe revolves around the sun at a mean distance of no less than 2,780 millions of miles, being about thirty times as great as the mean distance from the earth to the sun. The journey, though accomplished at a rate of more than three miles a second, is yet so long that Neptune requires almost 155 years to complete one revolution. Since its discovery, some forty years ago, Neptune has moved only through about one quarter of its path, and even since the time it was first casually seen by Lalande, in 1795, it has only had time to traverse half of its mighty circuit.

Neptune, like our earth, is attended by a single satellite; this delicate object was discovered by Mr. Lassell with his colossal

reflecting telescope shortly after the planet itself was known. The satellite performs its journey around Neptune in a period of but little less than six days. By observing the motions of the satellite we are enabled to determine the mass of the planet, and thus it appears that Neptune is about one twenty-thousandth part of the weight of the sun.

Our review of the planetary system closes when we have arrived at Neptune; whether any planets revolve around the sun in orbits beyond Neptune it is impossible to say. All we can assert is that no such planets have been seen, nor is there at present any good ground for believing in their existence. The negative evidence on the question is indeed very strong. We have in our chapter on the minor planets entered into a full discussion of the way in which these objects are discovered. It is by minute and diligent comparison of the heavens with elaborate star charts that these planets are found. But these inquiries would be equally efficacious in searching for a trans-Neptunian planet; in fact, we could design no better method to hunt for a trans-Neptunian planet than that which is at this moment in constant practice at many observatories. The labours of those who search for small planets have been abundantly rewarded by discoveries now counted by hundreds. Yet it is a most noteworthy fact that all these planets are limited to one region of the solar system. It is approximately true to say that all their orbits are included between those of Jupiter and of Mars. In one or two cases the orbits just reach beyond the path of Jupiter on the one hand, or inside the path of Mars on the other; but never has a planet been found in this way which goes out even as far as Saturn, and, of course, still less has any trans-Neptunian object been found. It has sometimes been conjectured that time may disclose perturbations in the orbit of Neptune, and that these perturbations may lead to the discovery of a planet still further, even though that planet be so remote and so faint that it actually eludes telescopic research. At present, however, and for generations to come, such an inquiry will hardly be within the range of practical astronomy. Neptune has not yet done more than travel through a quarter of its path since it was found

by Le Verrier and Adams. Its movements since then have no doubt been studied minutely, but it must at least describe one whole revolution before it would be feasible to construct from any perturbations of its path the orbit of an unknown and still more remote planet.

We have thus seen the planetary system to be bounded on one side by Mercury and on the other by Neptune. The discovery of Mercury was in itself a brilliant achievement of prehistoric times. The early astronomer who accomplished that feat, when devoid of instrumental assistance and unsupported by accurate theoretical knowledge, merits our hearty admiration for his untutored acuteness and penetration. On the other hand, the discovery of the exterior boundary of the planetary system is worthy of special attention from the fact that it was founded on profound theoretical learning, and verified by consummate instrumental and practical skill.

Though we here close our account of the planets and their satellites, we have still two chapters to add before we shall have completed what is to be said with regard to the solar system. A further and notable class of bodies, neither planets nor satellites, own allegiance to the sun, and revolve around him in conformity to the laws of universal gravitation. These bodies are the comets, and their somewhat more humble associates, the shooting stars. We find, in the study of these objects, many matters of interest, to which we shall proceed in the ensuing chapters.

CHAPTER XVI.

COMETS.

Comets Contrasted with Planets in Nature as well as in their Movements-Coggia's Comet-Periodic Returns-The Law of Gravitation-Parabolic and Elliptic Orbits-Theory in Advance of Observations-Most Cometary Orbits are sensibly Parabolic-The Labours of Halley-The Comet of 1682—Halley's Memorable Prediction-The Retardation Produced by Disturbance-Successive Returns of Halley's Comet-Encke's Comet-Effect of Perturbations-Orbit of Encke's Comet-Attraction of Mercury and of Jupiter-How the Identity of the Comet is secured-How to Weigh Mercury-Distance from the Earth to the Sun found by Encke's Comet-The Disturbing Medium-The Comets of 1843 and 1858-Passage of a Comet between the Earth and the Stars-Comets not composed of Gas of appreciable Density-Can the Comet be Weighed ?— Evidence of the Small Mass of the Comet derived from the Theory of Perturbation-The Tail of the Comet-Its Changes-Views as to its Nature-Carbon present in Comets.

IN our previous chapters which treated of the sun and the moon, the planets and their satellites, we found in all cases that the celestial bodies with which we were concerned were nearly globular in form, and many are undoubtedly solid bodies. All these objects possess a density which, even if in some cases it be much less than that of the earth, is still hundreds of times greater than the density of merely gaseous substances. We now, however, approach the consideration of a class of objects of a totally different character. We have no longer to deal with globular objects possessing a considerable mass. Comets are of the most irregular shapes, they are in large part, at all events, formed of materials in the utmost state of tenuity, and their masses are so small that no means we possess have enabled them to be measured. Not only are comets different in constitution from planets or from the other more solid bodies of our system, but the movements of comets are quite distinct from the orderly return of the planets at their appointed seasons. The comets appear sometimes with almost startling unexpectedness; they