XXXV111 NOTES. 



fanciful hypothesis propounded by him twenty-five years before (Phil. Trans, for 

 1693, vol. xvii. No. 195, p. 563), in which he supposed the Earth on which we 

 dwell to be a hollow shell, enclosing an interior smaller globe also supporting 

 human beings ; and that " in order to make that inner globe capable of being inha- 

 bited, there might not improbably be contained some luminous medium between 

 the balls, so as to make a perpetual day below." Then, supposing that the Earth's 

 compression near the poles of rotation must cause the outer shell to be thinner 

 there than at the equator, he considered that at certain seasons, and especially at 

 the equinoxes, the interior luminous and magnetic fluid seeks a vent through 

 fissures of the rocks composing the thinner crust near the poles ; and this 

 streaming forth of the bright fluid is, according to Halley, the phenomenon of the 

 aurora. Experiments with iron-filings strewed over a spheroidal-shaped magnet 

 (" a terrella"), served, he considered, to explain the direction of the luminous and 

 coloured rays of the aurora. " As every one has his own rainbow, so every ob- 

 server sees the corona at a different point." On the geognostic fancies of this 

 ingenious, and 'in all his magnetical and astronomical works, solid philosopher, 

 compare Kosmos, Bd. i. S. 178 and 425, Anm. 6 (English edition, p. 161 and 

 Note 136). 



( I6S ) p. 137. In the great fatigue of observing for many successive nights, 

 Oltmanns and I were sometimes relieved or aided by very trustworthy observers 

 the architect Herr Mampel, the geographer Herr Friesen, the highly informed 

 mechanician Nathan Mendelsohn, and our great geognost Leopold von Buch. 

 It is always a pleasure to me to name here, as hi all my earlier writings, those 

 who have kindly participated in my labours. 



( le9 ) p. 138. The month of September, 1806, was remarkably rich in mag- 

 netic storms. To exemplify this, I subjoin an extract from my journal : 



Sept. 1806 from 16 36 to 17 43 



If 16 40 19 2 



\\ 15 33 18 27 



8 I* 4 18 2 



I H 22 16 30 

 1? 14 12 16 3 



II 13 55 17 27 

 i 12 3 13 22 



The last was a small storm, and was followed by the greatest calm, which lasted 

 through the whole night and following morning until noon. On the 29th to 30th 

 Sept. 1806, at 10 h 20 m a small storm until ll h 32 m ,and then great calm until 

 17 h 6 m . 



