Size and Constitution of the Sun—The Solar Spots—Intensity of the Sun’s Light and Heat—The Sun’s Atmosphere—His influence on the Planets—Atmospheres of the Planets—The Moon has none—Lunar heat—The Differential Telescope—Temperature of Space—Internal Heat of the Earth—Zone of constant Temperature—Increase of Heat With the Depth—Central Heat—Volcanic Action—Quantity of Heat received from the Sun—Isogeothermal Lines—Line of perpetual Congelation—Climate—Isothermal Lines—Same quantity of Heat annually received and radiated by the Earth. THE sun is a globe 880,000 miles in diameter: what his body may be it is impossible to conjecture, but it seems to be a dark mass surrounded by an extensive atmosphere at a certain height in which there is a stratum of luminous clouds which constitutes the photosphere of the sun. Above it rises the true solar atmosphere, visible as an aureola or corona during annular and total eclipses, and probably the cause of the peculiar phenomena in the photographic image of the sun already mentioned. Through occasional openings in the photosphere or mottled ocean of flame, the dark nucleus appears like black spots, often of enormous size. These spots are almost always comprised within a zone of the sun’s surface, whose breadth measured on a solar meridian does not extend beyond 30-1/2° on each side of his equator, though they have been seen at a distance of 39-1/2°. The dark central part of the spots is surrounded by a succession of obscure cloudy envelopes increasing in brightness up to a penumbra, sometimes there are three or more shades, but it requires a good telescope to distinguish the intermediate ones. The spots gradually increase in size and number from year to year to a maximum, and then as gradually decrease to a minimum, accomplishing regular vicissitudes in periods of about eleven years, and are singularly connected with the cycles of terrestrial magnetism. From their extensive and rapid changes, there is every reason to believe that the exterior and incandescent part of the sun is gaseous. Doubts have arisen as to the uniformity of the quantity of heat emitted by the sun. Sir William Herschel was the first to suspect that it was affected by the quantity and magnitude of the spots on his surface; Professor Secchi has observed that the spots are less hot than the luminous part; and now Professor Wolf has perceived that the amount of heat emitted by the sun varies periodically with the spots every 11·11 years, or nearly nine times in a century, beginning at the commencement of the present one. He has discovered a sub-period in that of the spots, which no doubt has an effect on the quantity of solar heat. So the unaccountable vicissitudes in the temperature of different years may ultimately be found to depend upon the constitution of the sun himself.