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SUMTER, FORT 161 SUN the fort; at the end of that time it was silenced and in part demolished. Yet the garrison held on amid the ruins and in September beat off a naval attack ; and in spite of a 40 days' bombardment in Octo- ber-December, 1863, and for still longer in July and August, 1864, it was not till after the evacuation of Charleston itself, owing to the operations of General Sher- man, that the garrison retired, and the United States flag was again raised April 14, 1865; an event soon followed by the evacuation of Richmond and the Con- federate surrender. SUN, the center of our solar and plane- tary system, and one of the stars in the boundless sidereal universe. It is a hot self-luminous globe of enormous dimen- sions as compared with any of its planets, and the source from which they derive their heat and whatever life they bear; but considered as a star it is prob- ably only of moderate dimensions and brilliancy. Distance and Dimensions. — Its mean distance from the earth is about 93,000,- 000 miles, probably a little less, a dis- tance which a fast railroad train would traverse in about 250 years, sound (with its terrestrial atmospheric velocity) in about 14 years, a cannon-ball at 1,700 feet per second in about nine years, and which light flies over in about 500 sec- onds. Its diameter is about 866,500 miles, nearly 110 times that of the earth. With the earth at its center it would take in the whole moon's orbit and have plenty of room for another moon one and three-quarter times as far out as ours and still far inside its surface. Reduc- ing the scale so as to represent the sun by a globe 2 feet in diam.eter, the earth would be less than V4. inch in diameter and about 220 feet away, while the dis- tance of the nearest fixed star on this scale would be about 8,000 miles or the actual diameter of the earth. The sur- face of the sun is about 12,000 times that of the earth and its volume about 1,300,000 times that of our compara- tively small planet. Mass, Gravity, etc. — The mass of the sun is only about 332,000 times that of the earth, so that its density is only a little over a quarter of the earth's, or about 1.41 times as heavy as water. It is well to keep this in mind in thinking of the probable physical condition of the sun, when we remember that it is largely composed of iron. The attraction of the sun at its surface is about 27.6 times that of the earth at its exterior, so that a 200- pound man would vv^eigh about 5,520 pounds on the sun, a body would fall about 444 feet in a second, instead of 16, as here, and a pendulum which marks seconds here would vibrate more than five times per second there. Rotation and Axis. — The motion of the spots across the sun from E. to W. shows that the huge globe rotates regularly on an axis in a period of about 25.3 days, or rather this is the velocity at the solar equator. On each side the speed is slower, till in latitude 40° the period is more than 27 days. Much beyond this the ro- tation time is unknown, for the spots seldom extend beyond latitudes of 45° N. or S. The cause of this equatorial ac- celeration is as yet unexplained. The path of the spots also shows that the sun's equator is inclined to the plane of the ecliptic at an angle of about 1" 15', and that its axis points very nearly to a point half-way between the stars a Lyrse and Polaris. Photosphere. — The luminous surface of the sun directly visible in telescopes is called the photosphere (Greek phos, "bright"). It is probably a sheet of luminous clouds formed by the condensa- tion of substances which exist as gases in the hotter central mass of the sun. Un- der a moderate magnifying power it looks like rough drawing paper. With higher powers it looks something like snow flakes scattered over gray cloth. These flakes or grains are from 400 to 600 miles across, and are probably bright clouds floating in an atmosphere not so luminous. Near the edge of the sun the photosphere is much less brilliant than at the center, due to absorption of the solar atmosphere. Section of the Sun. — There is an inner nucleus; around it is the photosphere, rising at some places into faculae and depressed in others in spots. Immedi- ately above the photosphere is the "re- versing stratum"; above this is the scarlet chromosphere, with prominences of various forms and dimensions; and over, and embracing all, is the coronal atmosphere, fading gradually away into darkness. Eclipses oft'er exceptional op- portunities for studying the various phenomena of the sun that are outside the central nucleus. Spots and Faculse. — The most promi- nent feature of the sun's surface is the spots, some of which can almost always be seen except near the time of a sun- spot minimum. They are dark depres- sions in the photosphere, and consist of a central umbra with surrounding pen- umbra which is not so dark. This pen- umbra consists of radial filaments, and appears like the sloping sides of the sun- spot cavity, as if the photosphere were drawn down into the spot by inward cur- rents. In the central umbra there are also sometimes smaller, darker spots, called "Dawes's holes," from the name