Distinctly supplementary to the nebular theory are the remarkable researches, commenced in 1879, by Sir _George Howard Darwin_ (born 1845), son of Charles Darwin the great biologist. George Howard Darwin was born in 1845, at Downe in Kent, was educated at Cambridge, and studied for the law; but in 1873 he returned to Cambridge, where he became Plumian Professor of Astronomy in 1883. In 1879 he communicated to the Royal Society the first of his papers on tidal friction, which were summed up in his book on ‘The Tides,’ published in 1898. He finds that the tides act upon the Earth as a brake does upon a machine,—they tend to retard its rotation. Consequently, the day is growing longer, the Moon’s orbit is becoming enlarged, and its period of revolution is being lengthened. At present the day is about twenty-four hours long, and the month about twenty-seven days. The day, however, will be lengthened at a more rapid rate than the month, and in the remote future the day and month will both last fifty-five of our present days. The Moon will revolve round the Earth in the same period that the Earth rotates on its axis, and the two bodies will perform their circuit round the Sun as if united by a bar. Not only can we foresee the future of the Earth-Moon System, but we can also read the past. According to Darwin’s theory, the Earth, in the remote past, was probably rotating on its axis in a very short period, between three and five hours. The Moon must then have been much nearer us than it is now, and was probably revolving round its primary in the same period that the Earth took to rotate on its axis. The two globes, then gaseous, must have been revolving almost in actual contact. Had the month been even a second shorter than the day, the Moon must inevitably have fallen back on the Earth. As it was, the condition of affairs could not endure. The condition of the Moon resembled that of an egg balanced on its point. The Moon must either recede from the Earth or fall back upon it. The solar tide here interfered, and caused the Moon to recede from its primary until it reached its present distance of 239,000 miles.