If our desire of knowledge did not quicken as its sphere expands, or if man, so long as an eminence is unsurmounted, could lay himself down in peace, satisfied with the view of the vastness and variety which already stretch out beneath him, doubtless our task had now ended, and the volume of Astronomy might have been closed. But Desire, happily insatiable, has no confine on this side the Infinite; and no sooner have we reached the elevation of one thought or idea which resolves some large portion of the unknown, than ambition is fired afresh, and speculation never at rest, takes wing towards remoter regions.

In the present instance indeed there is every encouragement to further venturous inquiry. The Nebular Hypothesis, in its relations to the planetary system, may be termed complete;—it comprehends its beginnings, establishes those elements on which its duration depends, and exhibits the causes and mode of its ultimate transition into a novel form; and thus—surveying it from its commencement to its close—we are as if in possession of that primeval Creative Thought which originated our system and planned and circumscribed its destiny. Now, in reference to one of these epochs, our Hypothesis seems to hold equal connexion with the whole contents of the Heavens,—the epoch, viz.

2. Guided by the Genius, whose prophetic eye pierced the obscurity, which, until then, had concealed the arrangements we have just unfolded—we advance cautiously, but without dread, to take cognizance of still higher schemes.—The arguments which induced Herschel to pronounce on the connexion and motion of the constituents of binary systems, penetrate much farther, and intimate as a general law, that every cluster or unusual aggregation of orbs must be systematic, and probably united by common motion. If it is unlikely that the principles of random scattering would produce numbers of double stars, it is plainly as unlikely that anytriple or quadruple bodies should be found in the whole sky ; and this holds even where the stars are more separated, as in the case of the six principal constituents of the Pleiades, against whose fortuitous aggregation within that space—as Michell long ago calculated—there is a balance of probabilities of 500,000 to 1. The presence of a great law thus lays it on astronomers as a command that they watch these higher systems, take their measurements with every minuteness, and transmit them to posterity. The results must be brilliant, and they are already foreshadowed. In a triple star in Cancer (ζ), we are certain of a common motion, in which three suns seem to revolve around a common central point; and in another—ψ CassiopeiÆ — one sun probably revolves around a second, while the two in union—a sun and an associated sun, circulate around the third.

We resume our progress. The fact has been already established of the existence of clusters or firmaments, distinct from ours, sustaining an independent position, as individual constituents of creation. Let us now go forth into infinity among these firmaments, and ascertain their character.

The number of such masses is very great. In the northern hemisphere, after making all allowances, those, whose places are fixed, cannot be fewer than between one and two thousand; and you will have a good idea how plentifully they are distributed, by remarking that this is at least equal to the whole number of stars which the naked eye perceives in any ordinary night. These clusters, the general aspects of which I am now to describe to you, have very various appearances to the telescope. In many of them, individual stars are distinctly defined. As they become more remote, the distances or intervals between the stars diminish, the light also growing fainter ; in their faintest stellar aspect, they may be compared to a handful of fine sparkling sand, or, as it is aptly termed, star-dust; and beyond this we see no stars, but only a streak or patch of milky light, like the unresolved portions of our own surrounding zone. This is the state in which they are more properly called Nebulæ, and in which there is risk of confounding them with a singular substance not partaking of the nature of stars, but very common in our firmament.

Madam,

I have been induced to make the brief series of letters addressed to you, thus public, because of a regret which, I believe, is widely felt, that the discoveries made in recent years, throwing most unexpected light upon the constitution—present and remote—of the Stellar Universe, should longer continue comparatively unknown, and concealed amid the varied and massive collections of our 'Learned Societies. Unfortunately, I am not at present in a condition to bestow on these discoveries a shadow of original interest, so that, in description, my pen can have only a borrowed liveliness; but as the illustrious men who share the glory of having achieved such acquisitions for mankind, have, not unequivocally, shown a disinclination to the humbler task of reducing them into a popular system, we have only the choice of consenting, that matter of unusual importance shall remain unfitted to fulfil the best purpose of truth—which is to instruct and elevate the general mind—or to permit the work to be attempted by some one with pretensions no higher than my own.

Previous to the commencement of this century the facts and speculations about to engage us were unknown in science. Before then, the planetary orbits seemed to encircle all accessible space ; they had eiFectively constituted bounds to systematic enquiry, for astronomers had never adventured into greater remotenesses, having, like the people, gazed at the farther heavens with vague and incurious eye—content to admire their beauty and confess their mystery.