It is supposed that the Moon's motion has been rightly analyzed and that the true mechanism of its stability of revolution is fully and accurately known, at least so far as concerns the main forces involved. Out of many lesser factors, the few remaining to be accounted for are regarded as neither essential to stability nor inimical to it, but as giving rise to nothing more than minute irregularities, most of which are in the long run compensatory. The mathematical theory is so nearly complete that it enables astronomers to predict the Moon's place with a marvelous degree of accuracy. Indeed, for all purposes, except those of the utmost refinement, it may be said to be perfection itself. And yet, if we would adhere to the narrow line of truth, it must be admitted that there is something still lacking.