Our interest in the moon's physical characteristics never wanes. Her nearness to us has always fascinated astronomer and layman alike. Early users of the telescope were readily led into error regarding the general characteristics of the lunar surface; and it is easy to see why they thought the smooth level planes must be seas, and gave them names to that effect which persist to-day, as Mare Crisium, Mare Serenitatis and so on. We may be sure that no water exists on the moon's surface, although some astronomers think that solid water, as ice or snow, may still exist there at a temperature too low for appreciable evaporation. Perhaps water, seas, and oceans were once there, but their secular dissemination and loss as vapor have gone on through the millions of millions of years till even the moon's atmosphere appears to have vanished completely. At least there is much better evidence of absence of atmosphere on the moon than of its presence--not enough at any rate to equal a thousandth part of the barometric pressure that we have at the earth's surface. Frequent observations of stars passing behind the moon in occultation have satisfied astronomers on this point. We often say of the brilliant full moon, it is as bright as day. The photometer or instrument for accurate comparison of lights, their amount and intensity, tells a different story. Indeed, if the entire dome of the sky were filled with full moons, we should be receiving only one-eighth of the light the sun gives us, and it would require more than 600,000 average full moons to equal the light radiation of the sun. Heat from the moon, however, is quite different. Early attempts to measure it detected none at all, but with modern instruments there is little trouble in detecting heat from the moon, though measurement of it is not easy. Much of the moon's heat is sun heat, directly reflected from the moon, as sunlight is, but most of it is due to radiation of solar heat previously absorbed by the materials of the lunar surface. The actual temperature of the moon's surface suffers great variation. A fortnight's perpetual shining of the sun upon the lunar rocks would certainly heat them above the temperature of boiling water, if the moon had an atmosphere to conserve and store this heat; but the entire absence of such an air blanket probably permits the sun's heat to be radiated away nearly as fast as it is received, leaving the temperature at the surface always very low.