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SUN 162 SUN of their discoverer. The preceding is the description of what may be called a nor- mal spot before it begins to break up; but they are seldom of so regular a character. Frequently there are sev- eral umbrae Vidth a common penumbra, and there are sometimes streaks or "bridges" of the bright photosphere ex- tending clear across the whole spot. The umbra, is not always central, and the fila- ments of the penumbra are frequently twisted and curled into the most fan- tastic shapes, reminding one of swirling tongues of flame or smoke. The diameter of the umbra of a spot ranges all the way from 500 to 50,000 or 60,000 miles. The whole earth could be dropped into many of them without disturbing the edges. The penumbra surrounding some of the larger groups of spots has some- times measured as much as 150,000 miles in diameter. The depth of the umbra below the general surface of the photo- sphere is difficult to determine, but ac- cording to the best authorities may range from 500 to 2,500 miles. Sur- rounding the spots there are generally streaks of the photosphere which are much brighter than its general surface, sometimes extending nearly radially from the spot, and these are called the faculae (Latin fax, "a torch"). They are elevated ridges of the photosphere, and are much more prominent when near the edge of the sun, where they project up through a part of the atmosphere which dims the general photospheric. level. The faculse are not confined to the spot surroundings, but they are much more abundant there. They probably make surgings or upheavals, which are the surrounding accompaniment of sun- spot action. Life and Distribution of Spots. — They begin from insensible points, rapidly growing larger, but generally do not develop the penumbra till after the um- bra is well formed. The projection of a bright streak, or facula, across the nucleus of a spot often precedes its seg- mentation or breaking up into two or more. When a spot disappears it comes about by the encroachment of the photo- sphere, which seems to fall into and fill the cavity, leaving its place covered by a group of bright facula. In the vicinity of a spot the motion of phospheric and other matter is generally inward, toward and down into the center, and occasional- ly the latter is enough to be detected by the spectroscope. The duration of a spot ranges from a few days to occasionally more than a year, but is generally a month or two. It is a remarkable fact that their distribution is confined to two zones between 5° and 40° Theories of Sunspots. — The cause of spots is not yet satisfactorily made out, and among the many theories offered we can only note that of Faye, who con- siders them to be cyclonic, like our ter- restrial storms, caused by the forward drift of the equatorial photosphere, and the more probable suggestion of Young that they are depressions or sinks in the photosphere brought about by the di- minution of pressure below, which would accompany eruptions in the surroundings of the spot, the cooler, darker gases flow- ing down into the cavity thus formed. Certain it is that they are intimately as- sociated with eruptions and explosions on the surface or from below, but which is cause and effect, or whether both spot and eruption are caused by some outside influence (like the fall of meteoric mat- ter) is as yet undetermined. Penodicity of Spots. — The spots have, roughly speaking, a period of about 11 years, but it is very irregular and has not as yet received any satisfactory ex- planation. At the time of maximum the surface is never free from them, while at minimum none may be in sight for weeks at a time. Effect on the Earth. — The only certain connection between the spots and terres- trial phenomena is that with the earth's magnetism. The range of magnetic dis- turbance, or storminess, follows the sun- spot curve very closely, and individual outbursts on the sun are frequently ac- companied by simultaneous "magnetic storms" and brilliant exhibitions of the Aurora Borealis on the earth. The exact mechanical connection between the two is not yet known, but of its reality there is not the slightest doubt. Endless at- tempts have been made to connect almost every other phenomenon of terrestrial meteorology with the sunspot period, but with the single exception above men- tioned none has been satisfactorily es- tablished. Young's Reversing Layer. — Next above the photosphere comes a stratum of un- known thickness, discovered by Professor Young, containing the vapors of many of our terrestrial elements. At the time of a total eclipse, if the slit of a spectro- scope is kept just tangent to the disap- pearing limb of the sun, at the instant of the disappearance of the bright light from the photospheric background the light from this layer flashes out in the spectrum in the shape of bright lines, probably where the dark lines had before been. The point as yet unsettled is whether all the dark lines are thus re- versed, and with their relative intensity. Lockyer claims that this so-called lajner is of a considerable height, and that at