MAGNETIC DISTURBANCES. 129 



em or southern polar lights. Olav Hiorter and Celsius at Up- 

 sala were the first who, in the year 1741, and therefore be- 

 fore Halley's death, confirmed, by a long series of measure- 

 ments and determinations, the connection, which he had mere- 

 ly conjectured to exist between the appearance of the Aurora 

 Borealis and a disturbance in the normal course of the nee- 

 dle. This meritorious investigation led them to enter into 

 an arrangement for carrying on systematic, observations si- 

 multaneously with Graham in London, while the extraordi- 

 nary disturbances of variation, observed on the appearance 

 of the Aurora, were made subjects of special investigation 

 by Wargentin, Canton, and Wilke. 



The observations which I had the opportunity of making, 

 conjointly with Gay-Lussac, in 1805, on the Monte Pincio 

 at Rome, and more especially the investigations suggested by 

 these observations, and which I prosecuted conjointly with 

 Oltmanns during the equinoctial and solstitial periods of the 

 years 1806 and 1807, in a large isolated garden at Berlin, 

 by means of one of Prony's magnetic telescopes, and of a 

 distant tablet-signal, which admitted of being well illumina- 

 ted by lamp-light, showed me that this element of terrestrial 

 activity (which acts powerfully at certain epochs, and not 

 merely locally, and which has been comprehended under the 

 general name of extraordinary disturbances) is worthy, on 

 account of its complicated nature, of being made the subject 

 of continuous observation. The arrangement of the signal 

 and the cross-wires in the telescope, which was suspended in 

 one instance to a silken thread, and in another to a metallic 

 wire, and attached to a bar magnet inclosed in a large glass 

 case, enabled the observer to read off to 8" in the arc. As 

 this method of observation allowed of the room in which the 

 telescope and the attached bar magnet stood being left unil- 

 luminated by night, all suspicion of the action of currents of 

 air was removed, and those disturbances avoided which oth- 

 erwise are apt to arise from the illumination of the scale 

 in variation compasses, provided with microscopes, however 

 perfect they may otherwise be. In accordance with the opin- 

 ion then expressed by me, that "a continuous, uninterrupt- 

 ed hourly and half-hourly observation (Observatio Perpetud) 

 of several days and nights was greatly to be preferred to 

 isolated observations extending over many months," we con- 

 tinued our investigations for five, seven, and even eleven 

 days and nights consecutively,* during the equinoctial and 

 * When greatly fatigued by observing for many consecutive nights, 



