PREFACE.

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The only chapters in this treatise which requirecomment here, are the second and third, relating tothe motions of the Moon , and to her changes ofaspect:—

In Chapter II. I have given a very full account ofthe peculiarities of the Moon 's motions ; and notwith-standing the acknowledged difficulty of the subject, Ithink my account is sufficiently clear and simple to beunderstood by any one (even though not acquaintedwith the elements of mathematics) who will be at thepains to read it attentively through. I have soughtto make the subject clear to a far wider range of readersthan the class for which Sir G. Airy's treatise ongravitation was written, while yet not omitting anyessential points in the argument. In order to combineindependence of treatment with exactness and com-pleteness, I first wrote the chapter without consultingany other work. Then I went through it afresh, care-fully comparing each section with the correspondingpart of Sir G. Airy's Gravitation, and Sir J. Herschel'schapters on the lunar motions in his “ Outlines ofAstronomy." I was thus able to correct any errors inmy own work, while in turn I detected a few (mentionedin the notes) in the works referred to.

I have adopted a much more complete and exactsystem of illustration in dealing with the Moon ’smotions than either of my predecessors in the expla-nation of this subject. I attach great importance tothis feature of my explanation, experience havingsatisfied me not only that such matters should be very