ured by the most skilful artist; the information conveyed by the telescope is too definite to permit of speculation as with the other planets, yet not definite enough to solve the questions about which the students of astronomical works take most interest; and the information which astronomers have obtained from the moon's motions can only be appreciated when those motions are thoroughly analyzed, and it has not been found easy to simplify this analysis, that the general reader might fairly be expected to take interest in the matter.
The work before us is intended to remove this long-recognized want in the literature of astronomy. The time has come when this is practicable. The splendid photographs of Rutherford, of New York, and De La Rue, in England, supply the means of exhibiting truthfully the real nature of our satellite's surface. Mr. Proctor has been fortunate in obtaining from Mr. Rutherford permission to use three of his most effective photographs of the moon to illustrate the present work. Recent researches, again, into the processes which are going on within the solar system (so long mistakenly supposed to be unchanging in condition), suggest considerations respecting the past condition of the moon, at once bringing her within the range of speculation and theory. Telescopic observations, also more scrutinizing than those made of yore, and applied more persistently, begin to indicate the possibility at least of recognizing the signs of change, and perhaps of showing that our moon is not the dead and arid waste which astronomers have hitherto supposed her to be. The heat measurements of Lord Rosse also throw important light on the question of her present condition. And then, as respects those points which constitute the main scientific interest of our satellite, her motions under the varying influences to which she is subjected, Mr. Proctor has devoted here his full energies and the results of a long experience, to the endeavor to make clear, even to those who are not mathematicians, the considerations which, weighed and analyzed in the wonderful brain of Newton, supplied the means of demonstrating the theory of the universe.
On this important department of his subject, Mr. Proctor makes the following remarks in his preface: "In Chapter II. I have given a very full account of the peculiarities of the moon's motions; and, notwithstanding the acknowledged difficulty of the subject, I think my account is sufficiently clear and simple to be understood by any one, even though not acquainted with the elements of mathematics, who will be at the pains to read it attentively through. I have sought to make the subject clear to a far wider range of readers than the class for which Sir G. Airy's treatise on 'Gravitation' was written, while yet not omitting any essential points in the argument. In order to combine independence of treatment with exactness and completeness, I first wrote the chapter without consulting any other work. Then I went through it afresh, carefully comparing each section with the corresponding part of Sir G. Airy's 'Gravitation,' and Sir J. Herschel's
