76 



with the point of the heavens to which the south end of the dipping- 

 needle was directed at the time of observation. This point was 

 ascertained by means of observations of declination, horizontal 

 force, and vertical force, taken at the Greenwich Observatory during 

 the prevalence of the aurora by Mr. Brooke's photographic pro- 

 cess, the results of which were communicated to the author by the 

 Astronomer Royal. It is assumed that the magnetic declination and 

 dip at Cambridge differ from those at Greenwich at any given time 

 by certain constant quantities, whether the magnet be disturbed or 

 not. These constant differences were derived from the following 

 formulae : — 



V — V o = 0-142518\+ 0-1595481 

 D-D = 0'027713\ + 0-513523/, 



in which V and D are the declination and dip at a place not very 

 distant from Greenwich, V and D the contemporaneous declina- 

 tion and dip at Greenwich, X the longitude of the place ivest, in 

 seconds of time, and I the excess of its latitude in minutes above that 

 of Greenwich. These are merely formulae of interpolation by sim- 

 ple differences derived from the following data : — 



Lat. 



Greenwich ....51 28*6 

 Makerstoun. .. .55 34'7 

 Dublin 53 21*0 



The above are very accurate contemporaneous values of the decli- 

 nation and dip at the three places, and the formulae derived from 

 them will probably apply with considerable accuracy to any place 

 in the United Kingdom at any date not very remote from 1843. 

 For the Cambridge observatory V — V = + 3'"7 and D — D = 

 + 22'-0. 



The mean result from 24 observations of the position of the 

 corona is, that it was situated 5' further from the astronomical 

 zenith, and 1° 14' nearer to the meridian than the point of the 

 heavens to which the south end of the dipping-needle was directed. 



The places of the corona given by the different observations ex- 

 hibit considerable discrepancies, which are accounted for by saying, 

 that as the formation of the corona is merely an effect of perspective, 

 its position varies, since the streamers are not exactly parallel, with 

 the locality from which they rise ; also with any variation of their 

 direction at a given locality ; and, supposing the course of the 

 streamers to be somewhat curved in their ascent, it will vary with 

 the height to which they rise. Accordingly, as appeared to be the 

 fact, the corona would be continually shifting its position within 

 certain limits. 



Prof. Challis has made a similar comparison with observations of 

 the position of the corona of the same aurora made at Haverhill, at 

 Darlington, and at Bath ; also with observations at Whitehaven of 

 the aurora of Oct. 18, 1848, and of that of Oct. 24, 1847, at Cam- 



