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.