APPENDIX TO PART VII. 



EFFECT OF THE AURORA BOREALIS ON THE MAGNETIC DECLINATION, AND THE HORIZONTAL AND 

 VERTICAL FORCE AS OBSERVED AT THE GIRARD COLLEGE OBSERVATORY. 



There were in all 22 auroras recorded ; these, however, comprise only the brighter displays. Of 

 those observed, 7 occurred between May 30, 1840, and July 1, 1841 ; 1 occurred between July, 1841, 

 and July, 1842 ; 6 occurred between July, 1842, and July, 1843 ; and 1 between July, 1843, and 

 July, 1844. One is recorded in the last year, ending June 30, 1845. They are distributed over the 

 several months as follows : — 



January . . . . .2 

 February ..... 



March. 1 



April 2 



May 3 



June 2 



July 6 



August . ' . . . .3 



September 2 



October . . . . .0 

 November . . . . .1 

 December 



In the summer months there were 18, in the winter months 4. In reference to the hours of the 

 night, the phenomenon was visible on the average between 9-g- P. M. and 1 H P. M. 



Individual examination of the magnetic record during auroral displays. The time is local time, 

 counted for convenience' sake from midnight to midnight to 24 hours. 



I. 1840, May 29th. — 30th. As the twilight faded an aurora became visible. In the course of the 

 display there were moving pillars, flashes from a low segment of light in the north, and a beautiful 

 arch nearly or quite at right angles to the magnetic meridian. Pillars of aurora from 21'' 18"° to 22'' 

 2"", varying in brightness and position ; low segment of light to the north, continued throughout the 

 appearances ; at 22'' 5"" an arch forms from east to west ; streams of light, varying in brightness, 

 fading and reappearing from 22'' 20"° to about 23'' 10"" ; the brightest flash at 23'' 6"". From 18'' 54"" 

 the declination magnet commenced to move eastward (declination decreasing), reaching an extreme 

 position at 20'' 34"", difference from average position about 56 divisions or 19' ; the movement then 

 became westerly with smaller fluctuations till 22'' 39"", when it reached its westerly extreme of about 

 "71 divisions or 24' from the normal place ; the magnet reached a second easterly extreme at 23'' 44"" 

 of about 48 divisions or IT', at 1'' 24"" (30th) again a westerly extreme of about 7', and at 2'' 49"" an 

 easterly deflection of about 14' ; after this the needle returned gradually to its ordinary position. 

 About the time of the brightest flash the change (easterly motion) was very rapid, no extreme value, 

 however, was reached. When the arch formed, the position was nearly normal. The horizontal force 

 decreased steadily until 22'' 42"", when the readings fell beyond the scale ; a mininjum was reached 

 between that time and 22'' 52" of at least 0.016 (parts of the force) below the normal force. At the 

 time of the brightest flash the retrograde movement was in progress. The disturbance of the vertical 

 force commenced before IT'' 52"", at which time the force was a maximum; it then decreased very 

 rapidly, and finally moved off the scale after 22'' 2"". (The value of a division of the scale was not 

 ascertained.) 



II. 1840, July 4th. At 20'' auroral light in the N. N. W. about 10° aoove the horizon, at 22'' 

 very faint aurora still visible in N W. The declination was not at all affected. The horizontal force 



( S'^ ) 



