The ancients were unacquainted with the physical nature of comets–False ideas entertained by astronomers of the eighteenth century respecting the physical constitution of comets; comets regarded by them as globes, nearly similar to the planetary spheroids–Views of Laplace upon comets, compared by him to nebulae–Contemporary astronomers have confirmed these views and rectified the errors of the ancient hypotheses–Desideratum of science ; the rencontre of the earth with a comet or the fragment of a comet.

The question, What is a comet? examined in the preceding chapter, and which we reproduce as the heading of this Section, has been the subject of numerous hypotheses. It cannot, however, yet be considered as answered. But it has lately been attempted in an entirely new manner, and by a method least of all to be expected–that of direct investigation. The exposition of this method, and the considerations which have led to it, will be the object of this new chapter.

Let us commence by recapitulating the substance of what our previous enquiries and researches have already taught us.

The ancients, as we have seen at the commencement and in the course of this work, held notions concerning the nature of comets that were entirely hypothetical, and moreover contradictory. On passing their conjectures in review it is surprising, no doubt, to meet with ideas, to some extent, in conformity with the accepted facts of modern science.