632 



OBSERVATIONS OF THE AURORA BOREALIS OF NOVEMBER 17, 1848. 



" At is"" . 45™ the force was at its lowest value, being at that time below its usual value by about 

 one twenty-fifth part of the whole horizontal force. The force at l**" . 40° was but little above the 

 minimum, after which it increased very gradually till about noon of November 18 it nearly reached 

 the average value for the season and time of day." 



Table III. Approximate scale divisions of the Vertical Force Magnet about the time of the 

 Aurora Borealis of November 17, 1848, as observed at the Royal Observatory of Greenwich. 



Increasing readings denote an increase of force. One scale division is equal to the fractional 

 part 0,00067 of the whole vertical force. 



" Between gh. 27°° and 9\ 37™, the reading was somewhat greater, the light being off the paper. 

 Where two readings are included by a bracket, it is to be understood that the motion of the Magnet 

 was so rapid that both took place nearly simultaneously. Between l?*" . 35™ and 20*" . 5™ the reading 

 increased very gradually." 



A comparison of Tables II. and III. shews that the disturbance of the Horizontal Force was 

 much more considerable than that of the Vertical Force, and that both were generally below their 

 average values. 



CAMBnroGE Observatory, 

 November 27, 1848. 



J. CHALLIS. 



