average in the course of a single day, 7,500,000 meteors large enough to be visible to the naked eye are consumed in the earth's atmosphere, and about 400,000,000 meteors such as could be seen with a telescope of moderate power. Now, if we consider the latter set to be equivalent to the former, and assign a single grain as the weight of each meteor visible to the naked eye, we deduce fifteen millions of grains as the earth's daily increase of weight. This is rather less than a ton. So that in the course of about three years the earth's weight must increase (even on the very low value here assigned to a meteor's weight) by a thousand tons; and in the course of the three thousand years during which astronomy has been a science the earth's weight must have increased a million tons.' The moon's mass in the same time would be increased by about a sixteenth part of this amount.2 If, then, the earth alone, in circling once around the sun, gathers up tons of meteoric matter, it will be conceived how vast must be the weight of that meteoric matter (light though its particles be) which 1 This is a mere trifle compared with the earth's own weight, which is 6,000 millions of milllions of times greater. Indeed, it may easily be shown that the actual increase of the earth's radius in this interval of 3,000 years would be about the 70,000,000th part of an inch. 2 The dynamical effect of these increments would be an increase in the rate at which the moon circles around the earth. But the increase would be inconceivably minute; and the earth's mass must have been increased to a much greater extent if the actual observed excess of acceleration of the moon's motion over what the theory of gravity can account for, is to be explained in this way. Doubtless the retardation of the earth's rotation is the real explanation of the greater part of this acceleration, which comes therefore to be regarded as apparent only. in the course of a year has been within the earth's mean distance from the sun, and how enormous must be the combined weight of all the meteor systems, when this abundance of matter is continually maintained though the matter present in any one year has for the most part passed away before the next, and though year after year a proportion of the meteoric matter is withdrawn from orbital motion around the sun, and forced to form part either of his own mass or of the mass of some one of the orbs which attend upon him. But the considerations which now urge themselves upon our attention are far too numerous and too important to be discussed at the close of an essay like the present. I leave to another occasion the study of details which bear in the most striking manner on the economy of the solar system. I would particularly point to the fact that the new discoveries altogether change the aspect of the planetary scheme. The solar system as seen by Kepler and Newton may be compared to the trunk and main branches of a mighty tree, which modern discoveries present to us as adorned with lesser branches, twigs, and foliage, a tree still living and still growing. It may well be that as the study of astronomy proceeds we may recognise far more clearly and satisfactorily than now, the origin and the principles of the development of this mighty system. Fraser's Magazine for February 1871. 197 PROFESSOR TYNDALL'S THEORY OF ASTRONOMERS have not hitherto been fortunate in their theories respecting comets. These mysterious objects present so many perplexing appearances, and seem regulated by laws apparently so incongruous, that it has not been found possible to form an hypothesis which shall account even for the most important cometic characteristics. Although some comets are the largest objects in the solar system, surpassing even the sun himself in volume, yet the most brilliant comets are outweighed (perhaps many million-fold) by the tiniest asteroid, or even by the least of those minute satellites which make up the ring of Saturn. Obeying the attractive influence of the sun as submissívely as the most orderly of the planets, comets yet seem subject to other influences, repelling a portion of their substance with a force which seems a thousandfold more intense than the attractive influence of gravitation. Lastly, while we have the clearest evidence that a portion of the light we receive from comets is reflected solar light, exactly like that which we receive from the planets, we yet have equally decisive proof that comets are also self-luminous objects. So contradictory and perplexing are the peculiarities of these mysterious entities. It is clear that the problem presented by comets is one which requires for its solution a rare combination of powers and a widely extended range of research. The most profound acquaintance with physical laws is as necessary as a thorough grasp of the astronomical significance of cometic peculiarities. The ablest astronomer cannot hope to solve the problem by the unaided resources of his own science; nor can the physicist alone, however sound his knowledge, however clear his perceptions of the bearings of physical facts, or how great soever his skill in co-ordinating those facts into systematic hypotheses, hope to be more successful than the astronomer. The two, by working together, may at length succeed in mastering the problem which has, above all others, excited the curiosity of men of science, and more than any other has foiled their skill and ingenuity. It is pleasing, therefore, to find one of the most eminent physicists of our day turning his thoughts to the solution of this interesting problem. As Sir John Herschel remarked, when Professor Tyndall first began to investigate another well-known scientific crux, so may we say with reference to Tyndall's researches about comets :-'The subject is one eminently calculated to set one thinking, and it seems to have had that effect upon Professor Tyndall to an excellent purpose.' We must rejoice that he has been brought into contact with' comets, and still more so if he should be led to any satisfactory explanation' of their phenomena. It will be gathered that I am not able to recognise in the theory which I am about to describe the complete or even a satisfactory solution of the problem which has so long perplexed men of science. It was scarcely, indeed, to be expected that the class of researches which guided Professor Tyndall to the views he has put forward should lead at once to a solution of a problem of so much difficulty. Yet I believe that he has set us on the track of a useful and promising process of research, which, for anything that appears to the contrary, may eventually lead to the long-desired solution of that problem. Let it be premised that the fundamental idea running through all the noble series of researches carried out by Professor Tyndall, depends, if I understand his words and works aright, on the analysis of the ultimate particles of matter by the action of æthereal waves. Professor Tyndall has grasped, perhaps more fully than any living physicist, the fact that the undulations of the æther-that subtle medium whose existence is only known through its effects--afford the best if not the only available means of analysing what Newton called the more secret and noble works of nature within the corpuscles.' What science is waiting for is the Newton of the infinitely minute, and Professor Tyndall will one day, perhaps, be recognised as the Kepler of the great system of science, which is only awaiting the fulness of time to reveal itself to us in all its grandeur. However this may be, it is certain that his researches are gradually unfolding before 6 |