AURORA BOREALIS OF NOVEMBER 17, 1848. 625 



The variation of horizontal force was not registered from lo"" . 2™ to IS*", the disturbance carry- 

 ing the magnet out of the limits of the photographic paper. From 12'' the observations were made 

 independently of the self-registering process. I have assumed that from lo"" . 2" to 12'" the dis- 

 turbance followed the same law as from 12". Sg" to 14''.5'", when the phasnomenon reappeared in 

 a similar phase, and accordingly have taken 0,0890 to be the mean horizontal force reading in the 

 former interval. 



Let now X and Y be the undisturbed horizontal and vertical forces respectively, X' , V their 

 disturbed values at any given time, and ,r, y, the horizontal and vertical force readings at that time, 

 deduced by interpolation from Tables II. and III, the former divided by 10,000. Then 



X'= X- (0,108(5 - a?) X, V'= Y- (21,6 - y) 0,00067 Y. 



Y' . Y , 



Hence since— -,= tan of the actual Dip, and -— = tan 68". £5 , it is readily shewn that 

 X X 



the actual Dip = 70°. 43',6 - [3,06215] x + [9,88822] y, 



the numbers in brackets being the Logs of the coefficients of a: and y. The Dip at Cambridge is 

 assumed to be the value given by this formula, increased by D-D,,, or + 22',0. 



From the Declination ( V) and Dip (Z>), the distance Z' of the Magnetic Zenith from the 

 Astronomical Zenith, and its distance M' from the meridian are given by the expressions, 



Z'= 90"- D, sin M'= sin V cos D. 



The following are the results of the calculations which have been now explained. 



Zen. Dist. Zen. Dist. ol' Z — Z' 



I 



Greenwich 



Mean Time ^ ^ nr .^ v -.u 



1W8. Nov.17. of Corona. Mag=. Zenith. 



8\ 47°',1 is". 59' 20". 35' - l". 36' 



49,1 19 . 10 20 . 39 - 1 . 29 



53,1 18 . 44 20 . 38 - 1 . 54 



8 . 59,8 20 . 3 20 . 37 - . 34 



9 . 2,6 20 . 30 20 . 36 -0.6 



7,1 22 . 5 20 . ,38 + 1 . 27 



11.4 23 . 12 20. 36 + 2 . 36 



17.5 18 . 29 20. 42 - 2 . 13 

 30,1 20 . 27 20 . 26 +0.1 

 33,4 20 . 56 20 . 26 -H . 30 

 42,3 21 . 21 20 . 32 + . 49 



9 . 54,1 20 . 14 20 . 42 - . 28 



10 . 4,1 20. 20 . 34 - . 34 



8,3 20 . 48 20 . 28 H- . 20 



18.6 20. 46 20 . .30 +0. 16 

 10.21,2 19.48 20.31 -0.43 



11 . \,(\ (\t) . Hi .20 . 23 -1.7 



t),\ 21 . II 20 . 24 + . 47 



11.0 19.21 20 . 24 -1.3 

 12,3 20. 26 20. 24 +0.2 



14.1 22 . 7 20 . 24 + 1 . 43 

 1 5,6 22 . 46 20 . 23 + 2 . 23 



20, 1 



21 . 4.-i 20. 24 + 1 . 19 

 21 .6 20 . 24 + . 42 



11 .24,1 20 .6 20. 24 - O . 18 



Means 20 . 35,8 20 . 30,9 + . 4,9 6 . .TO,8 7 . 44,6 - 1 . 18,8 



