224 SIZE AND CONSTITUTION OF THE SUN. SECT. XXV. 



SECTION XXV. 



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 

 ^yith the Depth Central Heat Volcanic Action Quantity of Heat 

 received from the Sun Isogeothermal Lines Line of perpetual Con- 

 gelation Climate Isothermal Lines Same quantity of Heat an- 

 nually 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 on each side of 

 his equator, though they have been seen at a distance of 39. 

 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 



