130 COSMOS. 



solstitial periods — the importance of such observations at 

 these times being admitted by all recent observers. We soon 

 perceived that, in order to study the peculiar physical char- 

 acter of these anomalous disturbances, it was not sufficient 

 to determine the amount of the alteration of the variation, 

 but that the numerical degree of disturbance of the nee- 

 dle 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 results, deduced from 6000 observations, made from 

 the middle of May, 1806, to the end of June, 1807, the os- 

 cillation generally fluctuated only from one half of a gradua- 

 ted interval to the other half, amounting therefore only to V 

 12" '; in individual cases, and often when the weather was 

 very stormy and much rain was falling, the needle appeared 

 to be either perfectly stationary, or to vary only 0*2 or 0*3 

 of a graduated interval, that is to say, about 24 // or 28 // . 

 But on the occurrence of a magnetic storm, whose final and 

 strongest manifestation is the Aurora Borealis, the oscilla- 

 tions were either in some cases only 14/ and in others 38' 

 in the arc, each one being completed in from 1 J to 3 seconds 

 of time. Frequently, on account of the magnitude and in- 

 equality of the oscillations, which far exceeded the scale 

 parts of the tablet in the direction of one or both of its sides, 

 it was not possible to make any observation.* This, for in- 



Professor Oltmanns and myself were occasionally relieved by very 

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

 Priesen, the skillful 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 

 labors. 



* The month of September, 1806, was singularly rich in great mag- 

 netic disturbances. By way of illustration, I will give the following 

 extracts from my journal : 



|i Sept., 180G, from 4h. 36m. A.M. till oh. 43m. A.M. 

 22 « " « 4h. 40m. " " 7h. 2i 



23 " '< *< 

 24 



24 «' << ti 



25 



25. il " " 



26 



26 " " " 

 2T 



27 (( tt tt 



;m. 



The disturbance last referred to was very small, and was succeeded 

 by the greatest quiet, which continued throughout the whole night 

 and until the following noon. 



f» Sept., 1806, from lOh. 20m. P.M. till llh. 32m. P.M. 



