From any distant point on the earth’s surface either north or south of it, the visible portion of the auroral ring presents the appearance of an arch across the horizon. Arctic explorers, far within the Arctic Circle, see this arch to the south of them. In our latitudes it spans the northern horizon. Separate beams or streamers may be distinguishable or not, according to the brightness of the discharge or its distance from the point of observation. Combinations of beams constitute so-called “draperies.” [Illustration: PHOTOGRAPH OF LIGHTNING, SHOWING “BLACK” FLASHES. (_graph by F. Ellerman._)] [Illustration: ATMOSPHERIC ELECTRICITY INSTRUMENTS ON BOARD THE “CARNEGIE.” Left: Penetrating radiation apparatus. Right: active-content apparatus. Below: Arrangement for supplying potentials to electroscopes and ionization chambers, (_Carnegie Institution._)] Occasionally, at times of great solar activity, part of the ring actually overlies our Northern States, and the aurora then becomes a magnificent spectacle in this part of the world. The whole sky may be filled with the shifting streamers, along which travel rapid pulsations of light, so that the phenomenon then suggests strongly what it really is--a vast electrical discharge passing down through the atmosphere from outer space. When the observer is thus surrounded by the beams, they seem, on account of perspective, to converge toward a point south of the zenith, where they form a beautiful _corona_ or crown. The position of this crown depends upon the slant of the beams, which, as already explained, follow the lines of force. A brilliant aurora is always accompanied by disturbances of the magnetic needle, which moves about erratically, so that compasses can no longer be depended upon. At the same time there are strong “earth currents,” which interfere with the operation of telegraph lines. Observations with the spectroscope seem to show that the light of the aurora is chiefly due to glowing nitrogen, though the most prominent line in the auroral spectrum has sometimes been referred to an unknown atmospheric gas. The various colors seen in bright auroras, including reds, greens, and yellows, are believed by some authorities to depend upon the varying speed of the electrical discharge. Experiments with vacuum tubes show that nitrogen, especially, gives great changes of color with changes in the velocity of the discharge. Another interesting revelation of the spectroscope is that there is apparently, a faint auroral illumination in the sky at all times and in all parts of the world, the so-called “permanent aurora.”