THE 

 LONDON, EDINBURGH and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



SUPPLEMENT to VOL. XIX. THIRD SERIES. 



O 



LXXV. On an extraordinary Magnetic Disturbance recently 

 observed at Greenwich. By G. B. Airy, Esq., M.A., F.R.S., 



Astronomer Royal*. 



N the 25th of September of the present year, a most ex- 

 traordinary disturbance of the magnetic instruments was 

 observed at the Magnetic Observatory attached to the Royal 

 Observatory of Greenwich. The disturbed state of all the 

 instruments attracted the attention* of Mr. Glaisher (chief as- 

 sistant in the magnetic department), at an early hour of that 

 day, and he immediately commenced a series of observations 

 on all the instruments, at short intervals. After a time, the 

 disturbance became so small that the extraordinary observa- 

 tions were discontinued ; but it again increased, and observa- 

 tions were again immediately made with all the instruments, 

 and repeated as rapidly as it was possible for one observer to 

 do so. But for the promptitude and judgement displayed by 

 Mr. Glaisher on this occasion, the record of this disturbance, 

 unprecedented in this magnetic latitude, would probably have 

 been lost. The following statement will give an idea of the 

 magnitude of the disturbance : — that within eight minutes of 

 time, the declination needle changed its position more than 

 2£° (having passed in both directions the range of the obser- 

 ving telescope, which includes that angle) ; that the vertical 

 force was increased by more than l-40th of its whole value, 

 the instrument having then reached the extremity of its range; 

 and that the horizontal force was increased about l-30th of its 

 whole value. 



The following more detailed account is taken principally 

 from the abstract furnished by Mr. Glaisher. It is to be re- 

 marked, that the time throughout is Gottingen mean solar 

 time, civil reckoning : that the readings of the theodolite for 



• From a paper printed for private circulation, and kindly communi- 

 cated by the Author. 



PhiL Mag. S. 3. Vol. 19. No. 127. Suppl. Jan. 1 842. 2 L 



£0< 



