Josephus, appeared suspended over Jerusalem shortly before its fall. At least we have no certain record of any other comet nearer the time of the fall. Then follow returns on A.D. 141 March 25th, A.D. 218 April 6th (described as a terrifying spectacle, preceding the death of the Emperor Macrinus), A.D. 295 April 7th, A.D. 373 November 7th, A.D. 451 July 3rd. This comet came about the time of the defeat of Attila by Etius; it is referred to by Idatius, who says it was seen as a morning star in June, and an evening star in July. The Chinese annals accurately describe its course from the Pleiades through Leo, ending near Beta Leonis. A.D. 530 November, when it was described as very grand and terrifying, resembling a burning torch; A.D. 607 March, A.D. 684 October. This appearance is interesting, from a rough sketch in the Nuremberg Chronicle, that purports to represent it; there is however no proof that it is really contemporary; A.D. 760 June, very full Chinese record, enabling Laugier to confidently identify the comet as Halley's; A.D. 837 February 25th, taken by Louis le Debonnaire as a sign of his approaching death. A.D. 912 July, a return which til lately rested on computation only, no observation being known. A Japanese astronomer, Hirayama, has now found a record of its visibility in Japan from July 19th to 28th. A.D. 989 September 2nd; A.D. 1066 March 25th; this is the apparition that is associated with the Norman Conquest. The terror that it caused in England is illustrated by the manner in which it was apostrophised by Elmer, a monk of Malmesbury; "Venisti, multis matribus lugende; dudum est quod te vidi, sed nunc multo terribiliorem te intueor, patriae hujus excidium vibrantem." It is perhaps permissible to note of this same Elmer that he invented a flying machine, the wings being operated by his hands and legs, and launching himself from a high tower, flew for a furlong; but caught in a sudden gust and becoming panic-stricken, he fell headlong, and was lamed for life, a disaster which he ascribed to his having omitted to give his machine a tail. On the other side of the Channel William of Normandy took the comet as of good omen for himself, and one of his courtiers wrote the following doggerel lines upon it : : Caesariem, Caesar, tibi si natura negavit As is well known, the comet is portrayed on the Bayeux tapestry, and this is the oldest representation of it that is certainly authentic. Crude as it is, there are two features that are con firmed by modern photographs-the tail streamers, radiating like a fan from the nucleus, and the luminous masses which have the aspect of moving rapidly outwards. A group of Normans gazes at the comet in wonder "Isti mirantur stellam." In the adjoining panel of the tapestry Harold is represented quaking on his throne under the combined terrors of the comet, the landing of the Norsemen, and the threatened Norman invasion. A.D. 1145, April 19th. This return is of special interest, since the perihelion passage was on the same day as in the present year, and consequently the motion and behaviour of the comet are closely similar. Some interesting colloquial details are given by Hirayama. It was first seen about April 20th as a morning star; by May 9th its tail was 5° long; about May 15th it passed the sun, and became an evening star. The next day the chronicler says, "The tail was 5° long, directed towards the east; the end was concealed by clouds; I went out of the door and saw it." On May 17th the tail was 20° long. On June 4th the head was seen, but the tail had disappeared, to the astonishment of Moronaga, a friend of the chronicler. The tail reappeared on June 8th, and moonlight is stated to have been the cause of its disappearance earlier. We have, however, in modern times some undoubted cases of the disappearance of tails for a time. It was followed in Japan till June 18th, and in China till July 14th. It will scarcely be followed so long with the unaided eye at the present return. I have myself no doubt that the intrinsic lustre of the comet has greatly declined since the middle ages, though it is right to say that Dr. Holetschek, a great authority on the subject, takes an opposite view. The return of September, 1222, is one in which we (Mr. Cowell and myself) may justifiably take some pride, as we were the first to show that this grand object was Halley's comet; the much feebler object of July, 1223, had previously been taken for it. That of 1222 must have been a very striking sight; the Japanese say that the head was white, and as large as the halfmoon; the tail was red, 17° in length. The European records state that in August a star of the first magnitude appeared, very red, with a long tail pointing to the zenith. Compared with it the moon appeared as if dead. and seemed to have no more light. The fact that both in Europe and Japan it was compared to the moon shows what a splendid object it must have been. Historians also give a glowing account of its splendour in October, 1301, when it appeared in mid-September in Gemini, and went through Ursa Major to Corona and Hercules, being visible for forty-six days. It was seen in all parts of the known world, but, as in most of the ancient returns, the Chinese records are much the most precise; indeed without their aid it would have been quite impossible to carry back this long chain of identifications. Let us give them the credit they deserve for their patient, long sustained vigils, which have added so greatly to our knowledge of the history of this comet. We have now an array of some twenty-nine observed returns, many of them recorded as objects of great splendour. The first reflection suggested by them is the close touch that we are brought into with far-distant centuries, in being able to contemplate the very same body that has so often filled the world with wonder and admiration; but besides the sentimental aspect, there are, I think, some deductions of value with regard to the constitution of this and other comets. Dr. Johnston Stoney some years ago developed the theory of planetary atmospheres from the standpoint of the kinetic theory of gases; the gaseous molecules are moving with speeds of miles per second, hydrogen having the greatest speed, and the speeds of the others diminishing as their density increases. Now each planet has a certain speed which suffices to carry objects away from its surface. In the case of the sun it is 383 miles per second, for Jupiter 37, for the other giant planets upwards of 13, for the Earth 7, Venus 6, Mars and Mercury 3, the Moon 14 miles per second. An explanation is found of the fact that hydrogen is found in the sun and giant planets, but not in the smaller ones, its molecular speed being too high. The earth can retain the denser gases, but the moon cannot, and her airless condition is thus explained. Now there is no doubt, from what we know of the mass of comets, that their critical speed is much lower even than that of the moon; hence it is clearly impossible that they could permanently retain a gaseous envelope; that which we see surrounding them is not, therefore, of the nature of a permanent atmosphere, but is perpetually escaping from the head of the comet, and perpetually being renewed. The tail that we may see in Halley's comet to-day is a different one from what was seen a month ago. At every return for two thousand years it has been seen to eject a series of huge tails, which streamed away into space, and could not be recovered by it. Now there inust be some storehouse to contain all this gas, and the storehouse must be of a much denser nature than the gas, since it moves as though under gravitation alone, while the tail does not. And, seeing that we know that a close connection exists between gas, the comets of 1862, 1866, and Biela's comet with the Perseid, Leonid and Andromedid meteor showers respectively, and further that the meteors that have fallen to earth and have been chemically analysed have been found to contain much occluded especially hydrogen, which with its compounds is indicated in cometary spectra, it seems to me a most natural and probable deduction to draw that the reservoir containing the gas of comets' tails is a dense form of meteors; in fact, I should scarcely have thought it a matter of dispute, had not several well-known astronomers expressed doubts about the connection of comets and meteors. There is the further argument for the presence of a nucleus made of solid matter, that it appears to move exactly as if under the force of gravitation alone. The calculations of its motion are made on this assumption, and the difference between theory and observation in the time of its perihelion passage amounts to only three days in a period of some 27,000 days, showing that the action of non-gravitational forces on the head is barely sensible; but on the tail matter these repulsive forces far exceed gravitation, showing that the particles of the nucleus are much denser than those of the tail, and no doubt solid. I even venture to assert that the solid matter in the head of Halley's comet is not mere dust, but is in the form of pretty large lumps, at least several feet across, since otherwise I should expect the supply of gas to have been exhausted after a few returns. I think it is likely that the loss of gas occurs only when the comet is near the sun, the occluded gas being drawn out, either by the action of heat or some other exciting cause. When in the cold of outer space it probably sinks into a torpid condition and is devoid of envelopes. An exceedingly rare event is about to happen this month, which may throw some light on the constitution of the comet's head; I make out that this event, the transit of the comet over the disc of the sun, only happens if the perihelion passage falls in one particular half day of the entire year; that is, that one return in 700 or once in 50,000 years. Unfortunately the sun will be below our horizon when the comet crosses it, but astronomers in more favoured lands will be on the alert, notably at the Kodaikanal Observatory in India, whence Mr. Evershed writes to me that they are making preparations to photograph the sun in ultra-violet light, and in other methods that seem to give the best hope of success. Let us however consider the conditions, and we shall see that failure is quite likely; the comet will be 15,000,000 miles distant, or sixty times as far away as the moon. At that distance a lump of matter five miles in diameter would appear only one-fifteenth of a second across; this would be the very tiniest particle that would be separately visible; smaller particles might however be seen as a dusky patch, but only if they are closely congregated. There is no chance of seeing any of the gaseous envelopes of the comet against the brightness of the solar background. Even failure to see anything of the transit will teach us something, since we shall be able to fix superior limits to the size and density of the particles forming the nucleus. Since the tail of a comet points almost exactly away from the sun, it was at once seen that there was a possibility of our going through the tail at the time of the transit. The only element of doubt is whether the length of the tail will be sufficient to reach us; it will need to be 15,000,000 miles long, and Dr. Holetschek's researches show that it has only just attained this length at the more recent returns. Even if the tail does reach us, it is of such ethereal tenuity that it is quite doubtful whether we should be able to detect its presence when in the midst of it; there would be no contrast in this case, as when we see it from without on the black background of the sky, it would fill the whole heavens with a sort of diffused glare; something of the kind was recorded when we went through the tail of a comet in 1861 (it is instructive to see the apparent form of that comet when it was very near the earth; owing to perspective it appeared like a widely opened fan; we may look for a similar appearance if the tail of Halley's comet reaches us). Dr. Birkeland makes the suggestion that if we pass through the tail there may be a striking auroral display; this does not seem impossible, since the aurora is now thought to be due to the excitement of certain gases in our upper air by electrons emitted by the sun, of very similar nature to those supposed to form comets' tails. It is hardly likely that the presence of the tail would be sensible in any other way; arrangements have however been made by which any abnormal manifestation would be fairly sure to be detected. So much has lately been written about the physics of comets' tails that it is almost necessary to include some discussion of it. There is no question that there is some agency driving the tailparticles outwards from the sun much more potently than gravitation can pull them in; but as regards the nature of this action it is difficult to decide between three contending hypotheses. (i) That it is the pressure of light acting on the very tiny particles emitted by the head; this action is quite insensible compared with gravity in the case of large bodies, but when the |