the other systems of the universe. Should there be no link between them, it may be inferred, from the rotation of the sun, that the centre of gravity of the system situate within his mass describes a straight line in this invariable plane or great equator of the solar system, which, unaffected by the changes of time, will maintain its stability through endless ages. But if the fixed stars, comets, or any unknown and unseen bodies, affect our sun and planets, the nodes of this plane will slowly recede on the plane of that immense orbit which the sun may describe about some most distant centre, in a period which it transcends the powers of man to determine. There is every reason to believe that this is the case; for it is more than probable that, remote as the fixed stars are, they in some degree influence our system, and that even the invariability of this plane is relative, only appearing fixed to creatures incapable of estimating its minute and slow changes during the small extent of time and space granted to the human race. "The developement of such changes," as M. Poinsot justly observes, "is similar to an enormous curve, of which we see so small an arc that we imagine it to be a straight line." If we raise our views to the whole extent of the universe, and consider the stars, together with the sun, to be wandering bodies, revolving about the common centre of creation, we may then recognise in the equatorial plane, passing through the centre of gravity of the universe, the only instance of absolute and eternal repose.'
In the remaining section of Astronomy, Mrs Somerville treats, in succession, of the perturbations arising from the mutual actions of the primary and secondary planets—of the figure of the earth—of the phenomena of Precession, Nutation, and Libration—of the theory of the tides, and of the stability of the ocean; and the principles thus investigated are then applied to the determination of the figure of the earth, to the acquisition of standards of weights and measures, and to the rectification of chronological epochs. All these topics are treated with much sagacity and precision. The brief limits of the volume exclude all minute details, and the attention of the reader is arrested only by the leading doctrines and facts of astronomy. The condensation of thought and of language which is thus rendered necessary, may occasionally produce obscurity of expression, and ambiguity of meaning, In the mind of an ordinary reader; but when this does happen, it is never the fault of the author, but the Inevitable consequence of the absence of diagrams, and the limitation of space.
The next branch of physical science which comes under Mrs Somerville's review, is Acoustics, which is introduced by an account of the chemical and physical constitution of the atmosphere. The theory and phenomena of sound are briefly and distinctly expounded; but the curious and almost enchanting subject of the vibration of solid bodies, is discussed in little more than two pages. This omission, which we cannot but consider as a defect,
