NATURAL HISTORY of NORWAY. it 



the impreiiion of the humid air, and mounts and floats above 

 the clouds, whofe motion likewife renders it variable. Whilft the 

 air is dry, whether by the frofts of winter, or the heats of fum- 

 mer, no north-light is to be feen. But upon the weather's begin- 

 ning to break, either by a thaw after a fharp froft, or by rains after 

 heat, and when thefe are preceded by damp exhalations, the 

 north-light breaks forth, as a certain prognoftic of the change* 

 For thefe exhalations have then nearly the fame effect in the 

 atmofphere, as the aforementioned intrusion of the air into the 

 glafs-globe; propelling upwards the lighter ethereal air, when for 

 a time it appears like the purple coloured fluid iftuing from the 

 glafs-globe, till it is diflipated, or mixed again with the ambient 

 air. It is further obfervable, that the air near the poles is far 

 more denfe, and comprelTes more vehemently, as being repelled 

 with lefs violence, than that in the middle of the globe, where 

 the centrifugal power operates with a more dired and immediate 

 force *. 



Should this hypothecs, as indeed I know of no better, be ap- 

 proved by fuperior naturalifts, it will afford a very ready folution 

 of a difficulty, which clogs all other fyftems; namely, It is well 

 known among thofe people of the north, who have the beft op- 

 portunities of obfcrving thefe lights in the air, that the general 

 region of them is not due north, but rather in the north-well: 

 quarter of the fky. Is it afked how this comes to pafs? it may 

 be anfwered, that as the ignorant imagine the fun daily to run 

 from eaft to weft, the more intelligent know, that, on the con- 

 trary, the earth daily revolves from weft to eaft; thereby on one 

 fide a rarefadion may be caufed in trie air, and on the other a 

 condenfation. It is likewife obfervable and confonant to this, 

 that from fun-fet to a little paft midnight, the Aurora borealis is 

 ftrongeft, and to the beft of my knowlege not towards the 

 morning. Let others who have more fagacity, inveftigate this 

 matter farther. 



I muft ail the reader's pardon for dwelling fo long on this 

 particular, though I am not without apology, face it appertains 



* In locis polaribus vis centrifuga nihil de gravitate aeris tollit, cum in earn fub 

 S vatE h apparere minimum, prope polos maximum; quemadmodumob! 



^SS&a^T* qU0<1UC eVmCUnt ' Petr ' V? ' n ^^brock, Elementa 



Part L K 



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