After their discovery by Lector Vassenius, of Götheborg, in 1733, they could only be studied during total solar eclipses, until Lockyer and Janssen taught us, in the year 1868, how to observe them in full sunlight by means of the spectroscope. [Illustration: Fig. 22.—Quiet prominences of smoke-column type] [Illustration: Fig. 23.—Quiet prominences, shape of a tree. The white spot indicates the size of the earth] [Illustration: Fig. 24.—Diagram illustrating the differences in the spectra of sun-spots and of the photosphere. Some lines in the spot spectrum are stronger, others fainter, than in the photosphere spectrum. In the central portion, two reversals; to the right, two bands. After Mitchell] [Illustration: Fig. 25.—Spectrum of a sun-spot, the central band between the two portions of the photosphere spectrum. The spot spectrum is bordered with the half-shadows of the edge of the spot. After Mitchell] The quiet prominences consist almost exclusively of hydrogen and helium; sometimes they contain also traces of metallic gases. They resemble clouds floating quietly in the solar atmosphere, or masses of smoke coming from a chimney. They may appear anywhere on the sun, and their stability is so great that they have sometimes been watched during a complete solar rotation (for about forty days); this is possible only when they occur in the neighborhood of the poles, where they always remain visible outside the sun’s limb. Figs. 22 and 23 show several such prominences according to Young. [Illustration: Fig. 26.—The great sun-spot of October 9, 1903. Taken with the photo-heliograph of Greenwich in the usual manner. The spot is shown at mean level of the calcium faculæ. The two following photographs show a lower-level and a higher-level section through the calcium faculæ] Sometimes the matter of the prominences seems to fall back upon the surface of the sun between the smaller flames of fire which we have likened to blades of grass (Fig. 21). In most cases, however, the prominences appear slowly to dissolve. When their brilliant glow fades owing to their intense radiation, they can no longer be observed. The quiet prominences, which seem to float at heights of about 50,000 km. and at still greater heights, must there be almost in a vacuum. Their particles cannot be supported by any surrounding gases, after the manner of the drops of water in terrestrial clouds. In order that they may remain floating they must be pushed away from the sun by a peculiar force—the radiation pressure (see Chapter IV.). [Illustration: Fig. 27.—The great sun-spot of October 9, 1903. Photograph of the low-level calcium faculæ with the aid of the light of the calcium line H. The spot is not obscured by the faculæ—at least, not so much as in the following illustrations]