Shooting upward from the photosphere with the tremendous velocity of one hundred or more miles per second, can be seen at all times, by properly screening off the light from the photosphere, the vast solar eruptions known as the _prominences_. These are composed chiefly of hydrogen and calcium gas, though other elements also appear, especially near the bases of the prominences. Prominences are of two varieties, the quiescent, or cloud-like prominences, that float high above the solar surface for days at a time in some instances and resemble terrestrial clouds in form, and the eruptive, or metallic prominences, that dart up from the surface of the sun in an infinite variety of forms that may be entirely changed in the short interval of fifteen or twenty minutes. These eruptive prominences usually attain heights of thirty or forty thousand miles on the average, but _exceptional prominences reach heights of more than one hundred thousand miles and in a few rare cases have reached elevations of over five hundred thousand miles, or more than one-half of the solar diameter_. Prominences are the most spectacular and beautiful of all solar phenomena, with the possible exception of the solar corona, which is the outermost of all the solar envelopes and also the most tenuous. The extent of the corona is enormous. Its outer streamers extend usually to distances of several million miles from the center of the sun. Measurements of the coronal light during total eclipses of the sun have shown that its intensity is only about one-half that of full moonlight, and it seems almost impossible to devise methods for detecting it, except during total eclipses, on account of the extreme faintness of its light. The sun, it is now known, is surrounded by a strong magnetic field in addition to the magnetic fields that exist in sun-spots. The cycle of sun-spot change is attended by marked changes in many forms of solar activity. The frequency of outbursts of eruptive prominences, the brightness and form of the corona, magnetic storms and weather changes on the earth are all closely associated with the sun-spot cycle. The cause of this sun-spot cycle, with all the attendant changes in the general solar activity, and the source of the apparently limitless supply of solar energy still remain the two chief unsolved secrets of the sun. XVII THE SOLAR SYSTEM Our sun is but a star traveling through the universe. It is accompanied in its journey to unknown parts of space, that lie in the general direction of the constellation Hercules, by an extensive family of minor bodies consisting of the eight planets and their encircling moons, one thousand or more asteroids, numerous comets, and meteors without number, all moving in prescribed paths around their ruler.