Periodicity 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 layer is of a considerable height, and that at different heights in it, different lines, or the same lines with different intensity, will appear, in accordance with his theory that in the sun our chemical ele ments are dissociated and float at dif ferent levels in the solar atmosphere. This is one of the points to be settled by observation at future eclipses.
Chromosphere and Prominences or Protuberances.— Above the reversing layer and interpenetrating it, or possibly identical with it, if Lockyer's theory should prove true, is an atmosphere of permanent gases called the chromo sphere. Hydrogen is the most abundant of these gases, and out of this chromo sphere rise the wonderful prominences or protuberances which form so promi nent a feature of the sun's surroundings at the time of a total eclipse, and which can be investigated by the spectroscope at any time as they consist almost wholly of hydrogen. They are of all imagin able fantastic shapes, and frequently rise, and are sometimes seen to be rapid ly projected to the height of several hun dred thousand miles, and at enormous velocities. In such case they are almost
always seen to be connected with some active sunspot, and they are unquestion ably the results of the upheavals, erup tions, or explosions accompanying the surroundings of the spots. The chromo sphere itself all along the limb of the sun, as seen in the hydrogen lines in the spectrum, is not a smooth, flat layer, but consists of filaments like upward tongues of flames, and has been com pared to the appearance of a "prairie fire," though there is no actual combus tion going on.
Spectrum and Constitution.—Almost all our knowledge of the constitution of the sun has been revealed to us by the spectroscope. It has shown us that not only do many of our so-called chemical elements exist in the sun, but that its temperature is so high that they exist there in the form of gases. Among the elements identified are hydrogen, iron, titanium, calcium, manganese, nickel, cobalt, chromium, barium, sodium, mag nesium, and platinum, with a strong probability in favor of copper, palladium, vanadium, molybdenum, uranium, alu minum, cadmium, carbon, and lead. With the exception of carbon all the above are metals. (Hydrogen in its chemical relations ranks as a metal.) A few years ago it was considered that the late Dr. Henry Draper had shown the strong probability of the existence of oxygen in the sun, but the later investi gations, while not decisive, tend to nega tive this conclusion. In this connection Mr. Lockyer's views must be mentioned. He considers that none of our so-called chemical elements are truly elementary, but that they may all be decomposed into simpler constituents, and that many of them are so dissociated in the sun and the stars. The matter is still one of the most important unsettled questions in the domain of astro-physics. A full ex position of his views may be found in his "Chemistry of the Sun." The revela tions of the spectroscope in regard to the chromosphere, prominences, and so-called reversing layer have already been briefly mentioned. It has also been instrumen tal in revealing the enormous velocities accompanying explosions, and eruptions on the sun. At the limb of the sun we see these revealed by the telescope di rectly in the huge hydrogen prominences, but the spectroscope shows another com ponent of this velocity in the direction to or from us in the line of sight. In the spectrum of a spot, and of the facul round it, the lines are frequently broken and twisted into remarkable shapes, in dicating motion to or from us of the gases in question at enormous velocities. In some cases hydrogen has been shown to be rushing toward us with a velocity of 300 miles per second. Occasionally the spectrum of the nucleus of a spot has shown a down-rush of matter into the cavity.