17. Undoubtedly all true astronomers, taught caution and temperance of thought by the discipline of their magnificent science, abstain from founding such assumptions upon their discoveries. They know how necessary it is to be upon their guard against the tricks which fancy plays with the senses; and if they see appearances of which they cannot interpret the meaning, they are content that they should have no meaning for them, till the due explanation comes. We have innumerable examples of this wise and cautious temper, in all periods of astronomy. One has occurred lately. Several careful astronomers, observing the stars by day, had been surprised to see globes of light glide across the field of view of their telescopes, often in rapid succession and in great numbers. They did not, as may be supposed, rush to the assumption that these globes were celestial bodies of a new kind, before unseen; and that from the peculiarity of their appearance and movement, they were probably inhabited by beings of a peculiar kind. They proceeded very differently; they altered the focus of their telescopes, looked with other glasses, made various changes and trials, and finally discovered that these globes of light were the winged seeds of certain plants which were wafted through the air; and which, illuminated by the sun, were made globular by being at distances unsuited to the focus of the telescope.[7]
18. But perhaps something more may be founded on the ramified and straggling form which belongs to many of the nebulæ. Under the powers of Lord Rosse's telescope, a considerable number of them assume a shape consisting of several spiral films diverging from one centre, and growing broader and fainter as they diverge, so as to resemble a curled feather, or whirlpool of light.[8] This form, though generally deformed by irregularities, more or less, is traceable in so many of the nebulæ, that we cannot easily divest ourselves of the persuasion that there is some general reason for such a form;--that something, in the mechanical causes which have produced the nebulæ, has tended to give them this shape. Now, when this thought has occurred to us, since mathematicians have written a great deal concerning the mechanics of the universe, it is natural to ask, whether any of the problems which they have solved give a result like that thus presented to our eyes. Do such spirals as we here see, occur in any of the diagrams which illustrate the possible motions of celestial bodies? And to this, a person acquainted with mathematical literature might reply, that in the second Book of Newton's _Principia_, in the part which has especial reference to the Vortices of Descartes, such spirals appear upon the page. They represent the path which a body would describe if, acted upon by a central force, it had to move in a medium of which the resistance was considerable;--considerable, that is, in comparison with the other forces which act; as for example, the forces which deflect the motion from a straight line. Indeed, that in such a case a body would describe a spiral, of which the general form would be more or less oval, is evident on a little consideration. And in this way, for instance, Encke's comet, which, if the resistance to its motion were insensible, would go on describing an ellipse about the sun, always returning upon the same path after every revolution; does really describe a path which, at each revolution, falls a little within the preceding revolution, and thus gradually converges to the centre. And if we suppose the comet to consist of a luminous mass, or a string of masses, which should occupy a considerable arc of such an orbit, the orbit would be marked by a track of light, as an oval spiral. Or if such a comet were to separate into two portions, as we have, with our own eyes, recently seen Biela's comet do; or into a greater number; then these portions would be distributed along such a spiral. And if we suppose a large mass of cometic matter thus to move in a highly resisting medium, and to consist of patches of different densities, then some would move faster and some more slowly; but all, in spirals such as have been spoken of; and the general aspect produced would be, that of the spiral nebulæ which I have endeavored to describe. The luminous matter would be more diffused in the outer and more condensed in the central parts, because to the centre of attraction all the spirals converge.