Magnetic Disturbance on September 25th and 26th, ISil. 157 



cessity of recording the exact position of the bar at stated intervals of quick 

 recurrence ; and possibly a very light needle, read by a mirror, or by reflec- 

 tion from its own highly-polished side, and suited to follow more instanta- 

 neously than the declination bar the effects of rapidly-succeeding impulses, 

 might be found useful in exhibiting the effect of each impulse more distinctly. 

 An observer so circumstanced would note the time of those movements only 

 which should appear to him deserving of special notice. Such observations 

 might be made in addition to the readings of the magnetometers, which might 

 continue to be made at intervals as short as the strength of each observatory 

 will permit ; and, where it can conveniently be done, the readings of the ho- 

 rizontal and vertical-force magnetometers should be simultaneous. 



In a letter just received from Toronto, dated November 19, Lieut. Young- 

 husband states, that though the curve of the 25th of September shows in- 

 deed extraordinary fluctuations, they are not to be compared to tliose ob- 

 served on the night of the 18th November (184.1), when the declination 

 magnetometer ranged through above 86', and the horizontal-force magneto- 

 meter went beyond the scale in Gauss's method of observation ; a light held 

 in the prolongation of the scale, about one foot from its extremity (equal to 

 the length of about 200 divisions), was reflected from the mirror into the field 

 of the telescope: whence the force must have been dinmiished beloiv the 

 avyage more than ^th of its whole value : the greatest rapid change of force 

 •was equivalent to 0*03 of the total value, which was shown by a progressive 

 movement of tlie magnet during five minutes. A very brilliant aurora ac- 

 companied this disturbance. 



Third Postscript, Dec. 29th.— Whilst finally revising the last page of 

 this notice, the September returns from the magnetic observatory at the Cape 

 of Good Hope have arrived. Although the reduction of these observations, 

 for the purpose of subjoining them, would occasion an inexpedient delay in 

 tiiis publication, it is satisfactory to be able to state, that the remarivable 

 disturbance under consideration manifested itself in that southern latitude ; 

 that it arrested the attention of Lieut. Eardley Wilmot at an early hour of 

 the 24.th ; and tiiat it was followed by that officer and his detachment with the 

 utmost promptitude and assiduity. Extra observations, at the same intervals 

 as on term days, were commenced at 6^ 12"" 30" a.m. on the 2'tth, and con- 

 tinued to 10 A.M. ; resumed at 2^ 35'", and continued to 4 p.m. ; resumed 

 again at 6^ 5"" p.m., and continued without intermission for the succeeding 

 thirty hours. The epochs here spoken of are Gottingen mean time. All the 

 magnetometers were greatly affected. The greatest disturbance of the hori- 

 zontal force connnenced about 10 a.m. on the 25th, and attained its extreme 

 limit at 7'' 45'" i'.m. on tiie same day. The vertical-force magnetometer was 

 deflected out of the field of view at 6^ 30'" p.m. on the 24.th, and remained 

 so : the instrument being adjusted afresh to the needle, the latter was a"-ain 

 deflected out of the field at 2** 27"" 30* p.m. on the 25th. '^ 



The observations at tlie Cape may form a supplement to this notice, ac- 

 companied by observations of tlie same disturbance, expected to arrive by the 

 next overland mail, from the magnetic observatory at Simla in the Himalaya, 

 where, presuming it to have occurred, it is not likely to have escaped the in- 

 defatigable vigilance of Captain IJoileau, of the Hiiiigal Engineers, director 

 of that observatory. The September returns from the Van Dienien's Land 

 Observatory, conducted by Lieut. Kay, It.N., may be expected in February ; 

 about which tiiiu; we may also hope to receive accounts of the same date from 

 Captain Itoss, K.N., who intended to pass the last fortnight of September at 

 the Cliatham Islands, where he would establish liis magnetometers on shore. 



