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86 In 1879 appeared George H. Darwin's mathematical treatment of the theory of the tides, in which he demonstrated the real origin of the Moon, already explained in the first chapter of this volume.

Two English works which are accompanied by maps should here be mentioned, those of Neisen (1876) and Elger (1895). The former is by far the most complete work on the Moon in the English language, while the latter is a very convenient little volume for the amateur.

Three photographic atlases of the Moon upon a large scale have been begun within the last few years. One of these is by the Lick Observatory. It is upon a scale of 38.36 inches to the Moon's diameter. About three-quarters of the Moon's surface has been covered, but it is understood that the work has now been discontinued.

The second atlas is by Weinek. This is based chiefly on photographs taken at the Lick and Paris observatories. The Lick photographs are all enlarged just twenty-four times, and therefore represent the Moon on a scale of about ten feet in diameter. The Paris photographs are all enlarged to a scale of exactly four metres to the Moon's diameter, or 13 feet, 1.5 inches. Two views are given of each formation, one taken at sunrise and the other at sunset. Thus far 200 views have been published of the more interesting formations of the Moon. It is hoped some time in the future to publish 200 more, thus completely covering the whole lunar surface.

The third atlas is composed of the very artistic set of charts published by the Paris Observatory. These charts measure twenty-four by thirty inches and represent different portions of the Moon on various scales, ranging from 47 to 106 inches to the Moon's diameter. All the charts are taken near the sunrise or sunset phases. Forty-one have so far been issued, covering about seven-eighths of the lunar surface, at one phase or another, and the work is still progressing. The charts are not arranged in any particular order, about one-third of them being devoted to the mountain region to the north of Tycho. The others are scattered at various points over the surface, except along the eastern limb. Many of them have been reproduced upon a smaller scale by the Belgian Astronomical Society.

An interesting account of the process employed by Messrs. Loewy and Puiceux in the production of these charts is given in the British Journal of Photography for September 26, 1902, and reproduced in Popular Astronomy, 1902, X., page 502. The instrument employed is the well-known equatorial coudé of the Paris Observatory, in