Page:Popular Science Monthly Volume 53.djvu/488

470 slow-moving forms are white, lined with yellow, milky and difficult to define. Rose carmine is the next most noticeable color. In the rapid-moving arcs, crowns, wreaths and draperies, the center is usually yellowish, one extremity red, the other green. The red is almost always toward the lower part and also in the direction to which the ray moves, while the green is above and behind. For instance, if a ray darts down from a crown, the lower end will be red, the upper green. These colors will be very brilliant; when the red is very brilliant, the green is as intense. The red remains longest and fades last, when fog obliterates the aurora finally.

Our author, it will be seen, is a close observer, and furnishes reasons for all his deductions. He has discovered that the brilliance



of the colors bears a definite relation to the state of the atmosphere. In high latitudes. Sir John Franklin, McClintock, Weyprecht, and others aver that the coloring of the aurora was less strong when the air was very pure, and increased when it became foggy. The fine drapery forms are generally seen where seas are open in winter, free from ice, hence subject to fogs, as in Norway, Spitzbergen, and Newfoundland.

The light from auroras is very feeble; only a few lines of print can be read; while by the light of the full moon this is easy. The intensity of the aurora rarely exceeds the light of the moon in her first quarter, even in the arctic regions; this is corroborated by Parry, Kane, Hayes, Nordenskiold, and others. Auroras are less frequent in the full of the moon, paling in her effulgence, which drowns the