The first symptom of an aurora borealis is commonly a low arch of pale, greenish-yellow light, placed at right angles to the magnetic meridian. Sometimes rays cover the whole sky, frequently showing tremulous motion from end to end; and sometimes they appear to hang from the sky like the fringes of a mantle. They are among the most capricious of natural phenomena, so full of individualities and vagaries. To the glitter of rapid movement they add the charm of vivid colouring. It is strongly asserted that aurorae are preceded by the same general phenomena as thunder-storms. This was borne out by Piazzi Smith (late Astronomer-Royal for Scotland), who observed that their monthly frequency varies inversely with that of thunder-storms--both being safety-valves for the discharge of surplus electricity. Careful observers have, moreover, noticed a remarkable coincidence between the display of aurorae and the maxima of the sun's spots and of the earth's magnetic disturbances. Some have supposed that the light of the aurora is caused by clouds of meteoric dust, composed of iron, which is ignited by friction with the atmosphere. But there is this difficulty in the way, shooting stars are more frequent in the morning, while the reverse is the case with the aurora. The highest authorities have concluded, pretty uniformly, that aurorae are electric discharges through highly rarefied air, taking place in a magnetic field, and under the sway of the earth's magnetic induction. They are not inappropriately called "Polar lightnings," for when electricity misses the one channel it must traverse the other. The natives of the Arctic regions of North America pretend to foretell wind by the rapidity of the motions of the streamers. When they spread over the whole sky, in a uniform sheet of light, fine weather ensues. Fitzroy believed that aurorae in northern latitudes indicated and accompanied stormy weather at a distance. The same idea is still current among many farmers and fishermen in Scotland.