[AS.] A tool for boring holes, larger than those made by a gimlet. It has a handle placed crosswise, by which it is turned with both hands. The pod-auger and the screw-auger are the two principal kinds. The former has a straight channel or groove, while the latter has a twisted blade, by the spiral groove of which the chips are discharged.
Aurora Borea'lis, [L.] or Northern Lights. Luminous appearances which are seen in the northern quarter of the heavens. A muddy appearance of the sky in the direction of the north is the first indication of the approach of the aurora. This gradually resolves itself into a band of a dusky hue, in shape like part of a circle, stretching from the north towards the west, with its ends resting on the horizon, and surrounded by a continuous luminous arch of transparent white tinged with green. The arch is in a state of continual movement,.either rising and falling or swaying from east to west, and. starting from it, streams of light of brilliant and variegated colors shoot up towards the zenith. The most reliable measurements place the height of the aurora at from 45 to 100 miles above the earth. It is now regarded as certain that there is an intimate connection between the aurora and the magnetism of the earth, this being shown by the fluctuation of the magnetic needle during an auroral display, and also by the fact that the top of the luminous arch is found to be near the magnetic meridian. The aurora has been frequently observed to occur at both magnetic poles of the earth simultaneously. Aurorae are more frequent in North America than in the same latitudes in Europe. Thunder-storms and aurorae are connected with each other - the former being characteristic of the tropics, and the latter of the polar regions. In Norway, Siberia, and Lapland auroras enliven to some extent the snowy landscapes and brighten the long winter evenings, and they furnish much light duringthe protracted nights at the Arctic region.