XLV. Observations of the Aurora Uorealis of November 17, 1848, made at the 

 Cambridge Observatory. By the Rev. J. Challis, M.A., F.R.S., F.R.A.S., 

 Plumian Professor of Astronomy and Experimental Philosophy in the University of' 

 Cambridge. 



[Read November 27, 1848.] 



The observations I am about to lay before the Society, relate principally to the position of 

 the Corona in the splendid display of Aurora Borealis which occurred on the night of Nov. 17. 

 During thirteen years that I have resided at the Cambridge Observatory, there certainly has not 

 been so favourable an opportunity of observing the position of this critical point of the phasno- 

 nienon as on the recent occasion : and as the observations I took have enabled me to make a 

 comparison of the position of the Corona with the Magnetic Declination and Dip at the place of 

 observation more accurately than in any former instance that I am acquainted with, I have thought 

 them worthy, with their results, of being formally submitted to the notice of the Society. 



The observations were made partly by estimation of the position of the Corona with reference 

 to neighbouring stars, and partly by means of a small altitude and azimuth instrument, which was 

 constructed by Mr. Simms (Fleet Street, London), according to my directions, expressly for taking 

 observations of this kind. I call the instrument a Meteoroscope. It has a graduated azimuth 

 circle of four inches radius, and a portion of an altitude circle of the same radius graduated from 

 0° to 120°. An arm somewhat longer than the radius of the altitude circle, and turning about a 

 horizontal axis passing through the centre of the graduation of that circle, carries a bar eighteen 

 inches long, by means of which the observations are taken. To that extremity of the bar which 

 is turned towards the object observed, a rectangular piece is attached having one side horizontal, 

 and consequently the other movable in a vertical plane. The other end carries a plate in which 

 is made an eye-let hole one-sixth of an inch in diameter. An altitude is taken by observing through 

 the eye-let hole the coincidence of the object with the horizontal side of the rectangular piece, and 

 an azimuth by observing its coincidence with the vertical side. Both are taken simultaneously by 

 observing the coincidence with the angular point. The bar is set obliquely on the arm which 

 carries it, for the purpose of observing altitudes a few degrees beyond the zenith, for wliich purpose 

 also the graduation of the altitude circle extends beyond 90". When the object is near the zenith, 

 for convenience it is looked at through another eye-let hole made in a small plate standing at right- 

 angles to the larger plate, the object being seen by reflexion at a small niirror, the plane of which 

 is inclined at an angle of 4.0" to the direction of the bar. In this case the other angular point of 

 the rectangular piece is brought into coincidence with the object, care iiaviiig been taken by the 

 maker of the instrument that the direction of collimation should in tiie two cases be tiie same. The 

 bar and altitude circle may be readily turned together about the vertical axis, and the bar separately 

 about its horizontal axis of motion, and both may be quickly clamped as soon as the observation is 

 taken. The graduations are read off by verniers to single minutes. The instnuneiit has 11 tripod 

 stand, furnished with adjusting screws for the purpose of |ilacing the axis of nioliou vertical by 

 means of a small spirit-level, which is carried round witii the vertical circle. Tlie feet of the 

 screws rest in three angular grooves formed each by two plane faces, i)y applying the feet to which, 

 the instrument is instantly in position, the vertical adjustment of the axis having been ]>reviously 

 Vol.. VIII. I'aut V. 4 L 



