NATURAL HISTORY of NORWAY. 



navigation: for as in a dark, calm night, the courfe of a fifli 

 is perceivable by a long and increasing track of light upon the 

 water, fo the water, behind a fliip under fail, appears luminous 

 to a confiderable diftance. 



It is not at all times, however, that this igneous effulgence is 

 to be feen in the fea ; but it frequently happens at an approaching 

 alteration of the weather, and on the change of the winds to fouth- 

 weft, when the. faline particles of the fea are thrown into a kind of 

 fermentation. In like manner, the northern-lights do not always 

 appear, but only at particular feafons, when the faline corpufcles 

 of the air are agitated by a natural fermentation. But the proper 

 rationale of this fermentation, and afcent of the faline particles of 

 the fea and air, is beft known to naturalifts, whofe refearches 

 turn on things of this nature. However, it is a general obferva- 

 tion among expert northern navigators, and the fimermen who 

 live along the coaft of Norway, that when the north-light moftly 

 appears to the weftward, it is a prognoftic of a fouth-weft wind ; 

 which confirms the opinion of the naturalifls, that fome regions 

 of the air, as well as of the fea, abound in faline corpufcles more 

 than others, and thefe, at certain times, create a ferment, and 

 diffufe a light through the air. Although this moft frequently 

 prefages the above-mentioned change of weather, yet, there is 

 often a confiderable interval, before the change actually takes 

 place. It is however certain, that the cold regions of the air 

 contribute greatly to the change and boifteroufnefs of the wea- 

 ther; particularly when the north- light has a copper- tinge, a 

 violent ftorm, at weft and north- weft, may be certainly expected, 

 though the weather may for a week after continue favorable to 

 navigators, before the ftorm comes on. Of this I have feen 

 many inftances. 



In this fermentation of the air the cold is abated, and if it ex- 

 tends fo far as to rarify the air of the atmofphere, this is called 

 mild weather : And when, by the elevation of the inferior air, it 

 is the more compreffed againft that region, which is faturated 

 with nitrous exhalations, fo that the wind in the inferior air fets 

 the lower part of the cold region in fome motion, this caufes 

 thofe corrufcations in the air, which are called the north-light. 



In thofe years, when the winter is unufually fevere, thefe nor- 

 thern 



