MAGNETIC DISTURBANCES. 133 



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 illumi- 

 nated by lamp-light, showed me that this element of terres- 

 trial 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 sub- 

 ject of continuous observation. The arrangement of the 

 signal and the cross wires in the telescope, which was sus- 

 pended in one instance to a silken thread and in another to 

 a metallic wire, and attached to a bar magnet, enclosed 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. beinsj 

 left unilluminated by night, all suspicion of the action of 

 currents of air was removed, and those disturbances avoided, 

 which otherwise 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 opinion then expressed by me that " a continuous unin- 

 terrupted hourly and half-hourly observation (Observatio 

 Perpetua) of several days and nights was greatly to be pre- 

 ferred to isolated observations extending over many months," 

 we continued our investigations for 5, 7, and even 1 1 days 

 and nights consecutively, 68 during the equinoctial and solsti- 

 tial periods the importance of such observations at these 

 times being admitted by all recent observers. We soon per- 

 ceived that, in order to study the peculiar physical character 

 of these anomalous disturbances, it was not sufficient to de- 

 termine the amount of the alteration of the variation, but 

 that the numerical degree of disturbance of the needle must 

 be appended to each observation by obtaining the measured 

 elongation of the oscillations. In the ordinary horary course 

 of the needle, it was found to be so quiet that in 1500 re- 

 's When greatly fatigued by observing for many consecutive nights, 

 Professor Oltmanns and myself were occasionally relieved by very 

 trustworthy observers, as, for instance, by Mampel, the geographer 

 Friesen, the skilful mechanician Nathan Mendelssohn, and our great 

 geognosist, Leopold von Buch. It has always afforded me pleasure 

 to record the names of those who have kindly assisted me in my 

 labours. 



