PHYSICAL FEATURES. The surface of the moon, as seen from the earth, presents a most irregular grouping of light and shade. The dark portions were named by the earlier astronomers as seas, lakes, etc.. and still retain these names, although there is strong evidence against the supposition that the moon, or at least that portion of it presented to us. contains any water. The hrighter parts of the moon are mountaizions, as is proved by the fact of their casting shadows when the sun's rays fall upon. them obliquely, and also by the ragged appearance presented by the interior illuminated border of the moon, an appearance which can only be satisfactorily accounted for on the supposition that the surface of the moon is not level, in which case the higher portions will be illuminated some time before the light reaches the level park; and it is observed that as the illumination proceeds, bright spots start up in advance of it, and when the moon is on the wane, these same spots eontinue to shine for some time after the surrounding surface is immersed in gloom. The mountains occur either singly. when they are generally of a eireular form, and are called proicrR. or in groups, which are mostly annular. and form a sort of wall inclosing a deep depression or plain. in which are situated one or more conical mountains. The craters are not in frequently R or 10 miles in diameter, and some unetsam more than 100 miles across. 'file prin cipal mountains chain an altitude of about 20, 000 feet, according to a micrometric measure ment of the length of their shadows. The moon
everywhere presents traces of volcanic agency, but no active volcanoes have yet been discovered, nor is there any sign of recent volcanic action. Through the telescope, she presents a desolate appearance, without indications of animal or vegetable existence. She appears to be devoid of an atmosphere, or, if one exists, it must be of ex ceeding rarity. or else concentrated into c:n•itie.s within the moon's rocky surfaces. Probabilities are in favor of a low surface temperature,'not higher, perhaps, than that of ordinary ice. (For the influence of the moon upon terrestrial tides, see TIDES.) Reeent theorists hold that very large lunar tides, caused by the earth, existed many thousands of years ago, when the moon was still in a semi-plastie condition. This tidal theory can be made to account for the remarkable coin cidence of the lunar axial rotation period with that of her revolution round the earth. The best lunar map is that of Beer and Maedler,which was published in 1837. This map is based upon visual observations. and will undoubtedly be replaced in time by the results of photographic work. At present (1003) an elaborate series of lunar photographs is being published by the Paris Ob servatory; and a very good set of drawings from photographs taken at the Lick Observatory is being made by Weinek of Prague.