Surface The well-known resem blance of the full moon to a human face dis appears almost immediately if a glass —even the smallest opera glass —is trained on to it. When it is looked at through a moderately large telescope, the surface is seen to be broken up into mountain ranges and valleys as well as darker portions, which seem to be comparatively flat. A closer inspection made at more favor able times when the moon is not full so that the shadows cast by the sun can be well seen, puts these features clearly into view. The magnifi cent telescope in the Lick Observatory in Cali fornia brings the satellite so near that objects which might be seen with the naked eye if the moon were only 100 miles distant, can be dis tinguished on the surface under favorable cir cumstances.
The most marked feature of the lunar surface is the number of craters which appear in almost every region. These are cir cular rings with diameters ranging from half a mile to 100 miles, and with exterior walls sometimes as high as 20,000 feet—formations comparatively rare on the earth's surface where they seldom exceed a diameter of a very few miles with much lower walls. In some parts they are scattered in the wildest profusion, overlapping one another, smaller ones breaking into the walls of the larger and so crowded to gether that it is difficult to distinguish one from another. In many cases there is a central cone or a group of peaks which often rise as high as the walls of the ring and on which small craters can sometimes be seen. There are also lofty ranges of mountains ten to fifteen thou sand feet high, some peaks of the Lunar Ap penines rising to 20,000 feet.
Seas.— The so-called seas of the moon are simply portions of the surface darker in color than the average and very much less broken up by craters or mountain ranges. These form the main features of the face seen at full moon. They are crossed by thin lines known as rills or clefts which run in all directions, sometimes straight and unbroken for hundreds of miles, even intersecting ranges of mountains and craters and reappearing on the other side. These rills are generally two and rarely exceed 10 miles in width, their depth varying from 100 yards up. A curious feature of a different kind is an absolutely straight cut — the great Alpine valley — some 83 miles long which crosses a range of mountains and under low magnification looks as if some wan dering celestial body had grazed the surface.
White Rays.— The most puzzling feature of the surface consists in a series of white rays or streaks which radiate from a few of the principal craters in every direction. In their
brightness they mask all other shades of tint on the surface and seem to continue their course, sometimes for hundreds of miles, quite inde pendently of the nature of the country they cross. Prof. W. H. Pickering, however, who has studied the systems carefully, considers that their actual length has been much exaggerated and believes that the apparent length is due to lines of small craters from which they emerge. The most remarkable system is that starting from the crater Tycho, itself of a bril liant whiteness, and giving the whole region i the appearance of a globe cracked by internal pressure — a suggestion made by Nasmyth who actually cracked a glass globe in this way and obtained a striking resemblance.
Origin of the Formations.— The origin of these various formations has been the object of much speculation. That the craters and moun tain ranges came into existence after the cooling down of the outer crust and were produced by its contraction and by the enormous tidal dis turbances caused • by the earth seems a suffi ciently probable hypothesis. Objection has been raised to this view on account of the fact that terrestrial volcanoes all show the presence of large quantities of water and that the earth has comparatively few of such formations. But weathering action has undoubtedly had little effect on the moon's surface, while it has been a powerful factor in eliminating such features on the earth. Another theory, that the craters are of the nature of cracked bubbles like those which appear in cooling slag which contains gases, does not require the presence of water, but it has not met with any general acceptance. The rills or clefts are unexplained; some as tronomers incline to the idea that they are dried watercourses, others with greater probability that they are fissures produced in cooling. The white streaks or rays are considered by Mon sieurs Loewy and Puiseux, whose work on the moon accompanies a -big atlas of photographs taken lately at Paris, to be formed of volcanic dust or cinders shot out from the craters and carried• for considerable distances by currents of air before being deposited on the ground. Prof. W. H. Pickering inclines to the same theory. The rays were the last evidences of ac tivity before the body of the moon became cold and absorbed the small quantities, of air and water which at one time were present outside.