AURORA BOREALIS OF NOVEMBER I7, 1848. 623 



ward. Since these were made in the directions of the arcs m and z, the distance {M) of the 

 Corona from the meridian is m + ju, and its distance (Z) from the zenith is 2 + ^. 



When the Meteoroscope was used, the recorded altitude J, and azimuth B from East 

 towards South, were first corrected for index error by a and /3 respectively, and Z and M were 

 then obtained by the formulas, 



Z = 90''-(A + a), sin 31 = cos (S + /3) sin Z. 



The index corrections were deduced from Nos. (7), (10), (14.), (IS), and (27). The calculated 

 altitudes and azimuths of the stars, compared with the instrumental readings, gave the following 

 values of a and /3. 



No. (a) (13) 



(7) -40' +27°. 8' 



(10) - 2 +2.S. 1 



(14) + 5 +28.29 



(18) + 9 +25. 6 



(27) - 53 +22.48 



Mean - 16 + 25 . 18 



Respecting these values it is to be remarked, that the discordances between them are much 

 greater than might have been expected. From subsequent trials of the Meteoroscope I have 

 found that, without taking particular care in making the observation, the error in an arc of a 

 great circle may amount to 12'. Whether the discordances above arose from unsteadiness in the 

 support, the observations being made on the roof of a small out-building, on which several persons 

 were standing ; or from incautiously bending the collimating bar in the act of observing ; or, in 

 short, from inexperience in the use of the instrument, this being the first occasion of my using it 

 in a series of observations, I am unable to say. On replacing the instrument (Nov. 24), I ob- 

 tained from much more consistent values, the mean results - 5l' and + 23" . 34'. I have, how- 

 ever, considered it best to adopt the first determinations. 



I have now to explain in what manner the point of the heavens to which the South end of the 

 Dipping Needle was directed, which for the sake of brevity I call the Magnetic Zenith, was ascer- 

 tained. As we have no Magnetic Observatory here, this was done inferentially. I have assumed 

 that for any place in England, Scotland, and Ireland, the Westerly Declination of the needle (F) and 

 the Dip (D) may be given approximately by the formulae, 



V = V„ + a\ + bl 

 D = D„ + a'\ + b'l, 



K„ and D^ being the Declination and Dip at the Greenwich Observatory, X the Longitude of the place 

 Westward of Greenwich, I the excess of its Latitude above that of Greenwich, and o, h, a, h' certain 

 constants, which may be calculated by knowing the simultaneous values of V and D at Greenwich 

 and two other positions. From the published results of magnetic observations made in the year 1843 

 at Greenwich, and at the Observatory of Sir Thomas M. Brisbane (Makerstoun) ; and from a com- 

 munication, kindly made to me by Professor Lloyd, of the mean Declination at Dublin for the same 

 year as determined by 3^)00 observations, (viz. 27" . 9', 87,) and the Dip at Dublin as determined by 

 an elaborate scries of observations in September of 1843, (viz. 70". 4l',3), I have deduced very accu- 

 rate contemporaneous values of V and I), which with the Latitudes and Longitudes of the three 

 positions are here subjoined. 



Lung. West. 

 0"'.0",0 .. 

 10 . 3,5 .. 

 25 .4,0 .. 



From these data, were derived the following formulic, which probably may be applied at the present 

 time and for several years to come, with considerable accuracy to any place in the United Kingdom: 



4 L 2 



