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PREFACE.
freely illustrated, but that the illustrations should aimat correctness of detail, and (wherever practicable)of scale also. Some features, as the advance of theperigee and the retreat of the nodes, have, I believe,never before been illustrated at all.
In Chapter III. I give, amongst other matters, a fullexplanation of the effects due to the lunar librations.I have been surprised to find how imperfectly thisinteresting and important subject has been dealt withhitherto. In fact, I have sought in vain for any dis-cussion of the subject with which to compare my ownresults. I have, however, in various ways sufficientlytested these results.
The table of lunar elements will be found morecomplete than that usually given. In fact, in thistable, and throughout the work, my aim has been tohelp the student of the subject by supplying informa-tion not given, or not so completely given, elsewhere.It has always seemed to me that although in works onscientific subjects much of what is written must becommon property and many facts must be compiledfrom the writings of other authors, the main purposeof the writer should be to present results which he hashimself worked out and which are calculated to be ofuse to others. I doubt, indeed, whether any one isjustified in writing a treatise on science unless he hassuch a purpose chiefly in view.