It is the practice, in elementary and popular treatises on astronomy, to state merely the numerical results in giving data such as those embodied in the foregoing pages; and uninitiated readers, not knowing the means by which the figures are arrived at, are sometimes disposed to regard them with a certain amount of doubt or uncertainty. On this account we have thought it advisable to give, in as brief and concise a form as possible, the various steps by which these seemingly unattainable results are obtained. The data explained in the foregoing text are here collected to facilitate reference. Diameter of Moon 2160 miles 1/3·665 that of earth. Area 14,657,000 square miles 1/13·424 ” ” Area of the 7,328,500 square miles visible hemisphere Solid contents 5276 millions of cubic miles 1/49·186 ” ” Mass 73 trillions of tons 1/80 ” ” Density 3·39 (water = 1) 0·62 ” ” Force of 1/6 ” ” gravity at surface Mean distance 238,790 miles. from earth CHAPTER V. ON THE EXISTENCE OR NON-EXISTENCE OF A LUNAR ATMOSPHERE. At the close of the preceding chapter we stated that any force acting in opposition to that of gravity would be six times more effective on the moon than on the earth. But, in fact, it would in many cases be still more so; at all events, so far as projectile forces are concerned; for the reason that “the powerful coercer of projectile range,” as the earth’s atmosphere has been termed, has no counterpart, or at most a very disproportionate one, upon the moon. The existence of an atmosphere surrounding the moon has been the subject of considerable controversy, and a great deal of evidence on both sides of the question has been offered from time to time, and is to be found scattered through the records of various classes of observations. Some of the more important items of this evidence it is our purpose to set forth in the course of the present chapter.