626 PROFESSOR CHALLISS OBSERVATIONS OF THE 



The mode of observing by reference to a star is indicated by the letter s, and that by the Meteor- 

 oscope by the letter m. In taking the means, the observation at ll''. 4'",6 is excluded, the Corona 

 at that time being seen very obscurely after an interval of total disappearance. The great westerly 

 deviation given by that observation is, however, supported by the two that follow. 



The observations by stars taken separately, give -0° . 2l',8 for the value of Z-Z', and -1" . 50',7 

 for that of M-M'. The observations by the Meteoroscope give + 23',9 for the former, and 

 - 0" . 47',5 for the latter. 



The discordances in the positions of the Corona deduced from observation, are no doubt partly 

 owing to errors of estimation, or instrumental errors, and partly to the extreme difficulty of fixing 

 with precision on the centre of convergence of the Auroral streamers. But if these were the only 

 sources of discordance the distances from the zenith and from the meridian would be equally aflPected, 

 whereas the latter appear to be the more discordant. The fact seems to be, that the centre of the 

 Corona is contimtally shifthig its position. This may be owing to several causes. The formation 

 of the Corona is merely an effect of perspective, the apparent convergence of the streamers being due 

 to the immense height to which they rise. If the streamers were all parallel to a fixed straight line, 

 they would apparently converge to a fixed point. But the foregoing discussion, and facts that will 

 be hereafter mentioned, shew that they take, at least very approximately, the direction of the dipping 

 needle at the locality from which they ascend. Consequently the point of convergence will be 

 different for streamers, rising from different quarters. Again, the directions of the streamers may 

 vary by the same causes which produce the disturbances of the position of the dipping needle : and 

 this change of direction would of course alter the position of the Corona. Lastly, the course of the 

 streamers may not be rectilinear. The foregoing comparison appears to prove that the Corona is 

 decidedly more Westivard than the Magnetic Zenith, being less distant from the meridian than the 

 latter by 1° . l-i'. This is accounted for by saying that the streamers on rising from the Earth are 

 bent in a westerly direction. The apparent point of convergence would thus depend on the height 

 to which they rise, and would be continually varying. It is quite possible that streamers rising from 

 different quarters and to different heights, might apparently cross each other, and so form a fictitious 

 point of convergence. This explanation will, I think, sufficiently account for the discordances 

 observable in the foregoing results, and will serve also to shew why they exhibit no decided agree- 

 ment between the changes of position of the Corona and the changes of position of the Magnetic 

 Zenith. Such agreement may very well be veiled by the causes just mentioned. It seems to me, 

 however, that a general accordance of this nature is perceptible. As when the needle was most dis- 

 turbed, a large Easterly deviation of the South End was succeeded by a large Westerly deviation, 

 so a large deviation of the Corona to the East of its mean position was succeeded by a large Westerly 

 deviation : and as the changes of M' are more marked than those of Z , so the changes of M are 

 greater those of Z. 



For the purpose of farther illustrating the subject, I propose to add a discussion of a few 

 observations of the position of the Corona, made in the instance before us, and in one or two 

 others, in different parts of England. I have selected those of which the data seemed to be most 

 precise. 



Mr. Boreham of Haverhill informed me by letter that he found the Right Ascension of the 

 Corona of the Aurora Borealis of Nov. 17, to be l''. 58"". 3'; and its declination + 3l". 18', at g". 15"" 

 Greenwich mean time, the latitude of the place of observation being 52°. 5', and the longitude 

 1"". 46'. East. Hence I find by calculating as already described, 



Z Z' z - z M M' M-M 



22°. 56' 20°. 4.'.' + 2°. 11' 11". 18' 7°. 38' + 3°. 40' 



The differences in this instance are large, but not very different from those resulting from the 

 observation made at Cambridge at y''. 11™, 4. 



