BEFORE his death Sir George Darwin expressed the view that his lectures on Hill's Lunar Theory should be published. He made no claim to any originality in them, but he believed that a simple presentation of Hill's method, in which the analysis was cut short while the fundamental principles of the method were shewn, might be acceptable to students of astronomy. In this belief we heartily agree. The lectures might also with advantage engage the attention of other students of mathematics who have not the time to enter into a completely elaborated lunar theory. They explain the essential peculiarities of Hill's work and the method of approximation used by him in the discussion of an actual problem of nature of great interest. It is hoped that sufficient detail has been given to reveal completely the underlying principles, and at the same time not be too tedious for verification by the reader.

During the later years of his life Sir George Darwin collected his principal works into four volumes. It has been considered desirable to publish these lectures together with a few miscellaneous articles in a fifth volume of his works. Only one series of lectures is here given, although he lectured on a great variety of subjects connected with Dynamics, Cosmogony, Geodesy, Tides, Theories of Gravitation, etc. The substance of many of these is to be found in his scientific papers published in the four earlier volumes.

Introduction.

An account of Hill's Lunar Theory can best be prefaced by a few quotations from Hill's original papers. These will indicate the peculiarities which mark off his treatment from that of earlier writers and also, to some extent, the reasons for the changes he introduced. Referring to the well-known expressions which give, for undisturbed elliptic motion, the rectangular coordinates as explicit functions of the time—expressions involving nothing more complicated than Bessel's functions of integral order—Hill writes:

I propose to take advantage of the circumstance that this is the first of the lectures which I am to give, to say a few words on the Mathematical School of this University, and especially of the position of a professor in regard to teaching at the present time.

There are here a number of branches of scientific study to which there are attached laboratories, directed by professors, or by men who occupy the position and do the duties of professors, but do not receive their pay from, nor full recognition by, the University. Of these branches of science I have comparatively little to say.

You are of course aware of the enormous impulse which has been given to experimental science in Cambridge during the last ten years. It would indeed have been strange if the presence of such men as now stand at the head of those departments had not created important Schools of Science. And yet when we consider the strange constitution of our University, it may be wondered that they have been able to accomplish this. I suspect that there may be a considerable number of men who go through their University course, whose acquaintance with the scientific activity of the place is limited by the knowledge that there is a large building erected for some obscure purpose in the neighbourhood of the Corn Exchange.

George Howard, the fifth child of Charles and Emma Darwin, was born at Down July 9th, 1845. Why he was christened George, I cannot say. It was one of the facts on which we founded a theory that our parents lost their presence of mind at the font and gave us names for which there was neither the excuse of tradition nor of preference on their own part. His second name, however, commemorates his great-grandmother, Mary Howard, the first wife of Erasmus Darwin. It seems possible that George's ill-health and that of his father were inherited from the Howards. This at any rate was Francis Galton's view, who held that his own excellent health was a heritage from Erasmus Darwin's second wife. George's second name, Howard, has a certain appropriateness in his case for he was the genealogist and herald of our family, and it is through Mary Howard that the Darwins can, by an excessively devious route, claim descent from certain eminent people, e.g. John of Gaunt. This is shown in the pedigrees which George wrote out, and in the elaborate genealogical tree published in Professor Pearson's Life of Francis Galton. George's parents had moved to Down in September 1842, and he was born to those quiet surroundings of which Charles Darwin wrote “My life goes on like clock-work and I am fixed on the spot where I shall end it.”

The scientific work of Darwin possesses two characteristics which cannot fail to strike the reader who glances over the titles of the eighty odd papers which are gathered together in the four volumes which contain most of his publications. The first of these characteristics is the homogeneous nature of his investigations. After some early brief notes, on a variety of subjects, he seems to have set himself definitely to the task of applying the tests of mathematics to theories of cosmogony, and to have only departed from it when pressed to undertake the solution of practical problems for which there was an immediate need. His various papers on viscous spheroids concluding with the effects of tidal friction, the series on rotating masses of fluids, even those on periodic orbits, all have the idea, generally in the foreground, of developing the consequences of old and new assumptions concerning the past history of planetary and satellite systems. That he achieved so much, in spite of indifferent health which did not permit long hours of work at his desk, must have been largely due to this single aim.

The second characteristic is the absence of investigations undertaken for their mathematical interest alone; he was an applied mathematician in the strict and older sense of the word. In the last few decades another school of applied mathematicians, founded mainly by Poincaré, has arisen, but it differs essentially from the older school. Its votaries have less interest in the phenomena than in the mathematical processes which are used by the student of the phenomena.