Meteorological observations and essays / By John Dalton.

  • John Dalton
Date:
1834
Catalogue details

Licence: Public Domain Mark

Credit: Meteorological observations and essays / By John Dalton. Source: Wellcome Collection.

    98/280 (page 72)
    in the NW. quarter.—After 10h 30m grown faint; horizontal extent from WNW. to NE. by N. April 14th.—At Keswick, about midnight, or soon after, a still, horizontal light, altitude of the under edge 5°, of the upper edge 91; bearing of the centre NW. 4 N. GENERAL OBSERVATIONS. In order to determine the bearings of the middle or highest part of the arches of the aurora, I placed myself in a station where I had a distant object before me, in the direction of the magnetic meridian, and I always found the highest part in the same direction as this object ;—a deviation of 2 or 3° would in most cases have been very sensible.—Sometimes, to confirm the observ- ation, equal altitudes of the arches were taken on each side of the magnetic meridian, with the theodolite, and the horizontal angle divided into two equal parts, which gave the same bearing of the centre as the other method.—It does not, however, always hap- pen that the horizontal arch, especially when high, is perfect and complete. The streamers, or flashes, which pointed up, or perpendicular to the horizon, were only those in the magnetic meridian, as well south as north of the zenith. The altitude of the centre of the corona, when there was one formed, was taken with a quadrant and plummet, with as much exactness as the thing seemed to admit of. With regard to the needle of the theodolite, which was used to make the observations with, it is 34 inches long, and seems to move very freely upon its centre. I have often tried the effect of friction, by drawing it from its station, and then suffering it to vibrate till it settled, when it usually settled in the same station within one or two minutes, but I have sometimes observed it five minutes of a degree altered in such a case.