Sept. 4, 1884] 



NA TURE 



455 



I have had the good fortune to see other striking astronomical 

 phenomena. The first was the glorious comet of 1858, the last 

 was the transit of Venus ini8Sz ; but I have no hesitation in 

 saying that no phenomenon I have ever seen in the heavens, and 

 no spectacle that I have ever witnessed on the earth, has im- 

 pressed me so deeply and so profoundly as the great shower of 

 shooting stars in 1866. 



I was at that time astronomer to the late Earl of Rosse, at 

 Parsonstown, and in the autumn of the year I attended my first 

 meeting of the British Association at Nottingham. Fron- the 

 lips of my esteemed friend, Mr. James Glaisher, I learned that a 

 great shooting star shower was to be anticipated on the 12th of 

 November. The prediction could not be put forward with all 

 the confidence that we have when the almanac foretells an 

 eclipse. It was rather a venture, by which an important theory 

 was to be put to a severe test. 



On the ever-memorable night I was occupied as usual in 

 observing nebulae with the present Earl of Rosse at the 

 great reflecting telescope. In the early part of the evening 

 the sky was clear, and the night was dark ; but no unusual 

 phenomenon occurred until about ten o'clock. I was at that 

 moment watching a nebula at the eye-piece, when I was startled 

 by an exclamation from the assistant by my side. I looked up 

 just in time to see a superb shooting star stream across the 

 heavens. Soon came another star, and then another, and then 

 in twos and in threes. We saw at once that the prediction was 

 about to tie verified. We ceased the observations with the 

 telescope and ascended to the top of the wall, which forms one 

 of the supports of the great telescope. This position commanded 

 an extensive view of the heavens, and from it Lord Rosse and 

 myself, on a beautiful starlight night, witnessed that gorgeous 

 display of celestial fireworks which has gi en fresh impetus to 

 astronomy. 



It was not merely the incredible number of the shooting stars 

 that was remarkable. They came no doubt in thousands which 

 no man could number, but what was especially to be noticed was 

 the intrinsic brilliancy of each individual star. There were 

 innumerable meteors that night any one of which would have 

 elicited a note of admiration on any ordinary occasion. As the 

 night wore on and the constellation of Leo climbed up from the 

 east, then the display exhibited a very interesting and character- 

 istic feature, for, as each shooting star was projected across the 

 sky, the track which it followed was invariably directed from the 

 constellation of Leo, nay, even from a particular point in that 

 constellation. So marked a property of the shower suggests an 

 appropriate name, and accordingly this particular group of 

 shooting stars bears the not unpleasing name of the " Leonids." 

 It is easy to demonstrate that the apparent radiation of the 

 meteors from a point is only the effect of perspective. They are 

 really moving in parallel lines. Those parallel lines have a 

 vanishing point, and that point is the radiant in the constellation 

 of Leo. As we stood on the walls of the great telescope we saw 

 the true character of the radiant most beautifully demonstrated. 

 Those meteors which appeared close to the radiant pursued a 

 track which was greatly foreshortened. A few that were actually 

 at the radiant, or very close to it, had no visible track at all, they 

 merely shone like a very rapidly variable star, which rose from 

 invisibility to brilliancy, and then again declined to evanescence, 

 all within the space of a very few seconds. In these exceptional 

 cases we viewed the track of the stars "end on." They were, in 

 fact, coming straight at us, but fortunately there was a kindly 

 screen which shielded the earth that night from the awful meteoric 

 tempest. Each one of those meteors hurries along with a 

 velocity truly appalling ; it is more than a hundred times swifter 

 than the swiftest bullet that was ever fired from a rifle. It is 

 really the demoniacal impetuosity of this velocity which is the 

 source of the earth's safety. The meteor moving freely through 

 space suddenly plunges into our atmosphere. Instantly a 

 gigantic resisting force is aroused. The velocity of the meteor 

 is checked, and the energy stored in that velocity is transformed 

 into heat. That heat is enough to raise the body red hot, to 

 raise it white hot, nay, even to drive the solid mass into a streak 

 of harmless vapour. Of all the countless myriads of shooting 

 stars which went to their destruction on that night, not one 

 single particle has ever been recovered. These facts, when 

 placed in the crucible of the mathematician, conduct him to 

 a solution of the problem as to the nature of the great shoot- 

 ing star shower. It is to be remembered that the law of gravi- 

 tation determines the movements of these bodies. The meteor, 

 ere its disastrous collision with our atmosphere, must have 



been traversing the solitudes of space in an elliptic path with 

 the sun in one of the foci. This is as true of a meteor the 

 size of a grain of sand as it is of the earth or the planet 

 Jupiter. The astronomer then approaches the question with the 

 knowledge that the orbit of the meteors is an ellipse (or at all 

 events one of the conic sections), but what the particular ellipse 

 is must be decided by an appeal to the actual observations. The 

 facts are simple enough : we note in the first place that the shower 

 took place on the 1 2th of November, but on the I2th of Novem- 

 ber in each year, or on any other fixed date, the earth is always 

 at a particular point of its annual journey round the sun. The 

 stream of meteors must therefore pass through that particular 

 point of space, and hence the search for the orbit is narrowed, 

 for only ellipses which pass through this particular point can fulfil 

 the conditions of the question. Another clue is afforded by the 

 position of that point in Leo from which all the meteors seemed to 

 radiate. The mathematician sees how to fit the ellipse so that 

 it shall give the proper radiant. And now the question has been 

 narrowed almost to the last point. One more appeal to observ- 

 ation and the ellipse will be absolutely known. All we must 

 now learn is how long the swarm of meteors takes to complete the 

 circuit of its mighty path. To answer this question profound 

 historical research has been made by Prof. Newton, and a 

 mathematical research has been made which has given additional 

 lustre even to the name of Adams. The great showers of meteors 

 have been shown to have occurred at intervals for the last 1,000 

 years. The earliest record was in the year 902, on the occasion of the 

 death of the Moorish king Ibrahim-bin- Ahmad. An old chronicler 

 describes how the event was solemnised in the heavens no less 

 than on the earth ; he tells us how "that night there were seen 

 as it were lances, an infinite number of stars which scattered 

 themselves like rain to right and left, and that year was called 

 the year of the stars." We now know that this exhibition was 

 not, as the old chronicler thought, a miraculous compliment to 

 the memory of the deceased prince, it was really only a showe 

 of the Leonids, such a shower as appears every thirty-three years, 

 such 'a shower as appeared in 1866, such a shower as may !>■_- 

 anticipated in the year 1899. 



By these researches the path followed by the Leonids has been 

 completely determined. The plane of the ellipse, and every 

 circumstance of its position, and its proportions have been reduced 

 to numerical accuracy. The shoal of meteors pursue their path 

 unseen by any astronomer, but the mathematician knows 

 precisely where they are at this moment, and at every moment. 



This point being gained a great discovery was made by 

 Schiaparelli in 1866. About that time a comet was seen, this comet 

 was duly observed, and the path which it followed was computed. 

 There was nothing very remarkable about the comet, and it 

 would not now be much remembered save for one most extra- 

 ordinary circumstance which Schiaparelli was the first to proclaim. 

 Like the shoal of meteors this comet also revolves in an elliptic 

 path around the sun. This is a mere consequence of the law of 

 gravitation and calls for no remark, but the fact that the two 

 ellipses lie in the same plane is a very remarkable coincidence 

 which could not be overlooked. When we further come to see 

 that the two ellipses are of the same size and shape, when we 

 see that they are placed in the same position, when we see, in 

 fact, that the ellipse which is the orbit of the shooting stars is 

 identical with the orbit of the comet, then we have obtained a 

 result which ranks as one of the most striking astronomical 

 discoveries that this century has witnessed. 



The Leonids therefore travel through space precisely in the 

 track of the comet of 1S66. The question at once arises of the 

 relation of the shoal of meteors to the comet. Is the shoal of 

 meteors one thing and the comet another thing, and do both 

 these things happen to be travelling in the same orbit without 

 any necessary connection, or are we to suppose that the two 

 objects, if not actually identical, are at all events very intimately 

 connected ? These are problems which, in the present state of 

 our knowledge, it seems difficult to solve. I shall only lay down 

 one or two principles which may help us to form a conclusion. 



Whatever be the nature of comets, or the materials of which 

 they are composed ; whether they be faint or bright, large or 

 small, periodic or parabolic ; one fact is certain, their masses are 

 all extremely small in comparison with their great dimensions. 

 I shall indeed, at a later part of this lecture, show that comets 

 seem to be almo-t imponderable when compared with the great 

 masses of the sun and of the planets. The great bulk of a comet 

 necessarily implies that many parts of it are at a considerable 

 distance from its centre of mass. Hence for a double reason the 



