Scientific Lectures. 169 



from the greater densit} 7 of the matter, or the greater velocity of its 

 motion, after the same manner as we see the effluvia of electric 

 bodies, by a strong and quick friction, emit light in the dark — to 

 which sort of light this seems to have a great affinity. 



" This being allowed, I think we may readily assign a cause for 

 several of the strange appearances we have been treating of, and for 

 some of the most difficult to account for otherwise ; as, why these 

 lights are rarely seen anywhere else but in the north, and never, that 

 we hear of, near the equator ; as also, why they are more frequently 

 seen in Iceland and Greenland, than in Norway, though nearer the 

 pole of the world. For the magnetical poles, in this age, are to the 

 westward of our meridian, and more so of that of Norway, and not 

 far from Greenland ; as appears by the variation of the needle, this 

 year observed, full 12° at London to the west. 



"And whereas in this appearance (and 

 perhaps in all others of the kind), those beams which arose near the 

 east and west, were farthest from the perpendicular, on both sides 

 inclining toward the south, while those in the north were directly 

 upright — the cause of which may well be explained by the obliquity 

 of the magnetical curves, making still obtuser angles, with the 

 meridian of the terella, as they are farther from the poles. 



* " But whatever may be the cause of it, 



if this be not, I have followed the old axiom of the schools, Entia 

 non esse temere neque ahsque necessitate midtipliccmda." 



The fact that the earth's magnetic and geographic poles do not 

 coincide, at once leads us to suspect that the needle cannot, on the 

 meridians point directly north and south. Its departure from this 

 direction actually exists and was no doubt the earliest experience of 

 those who first made the great discovery of the directive property 

 of a suspended magnet. The first recorded use of this most valuable 

 discovery appears among the Chinese, where we find that " an appa- 

 ratus of this kind (called fse-nan — indicators of the south) was 

 presented during the dynasty of the Tschen, 1,100 years before our 

 era, to the ambassadors of Tonquin and Chochin-China, to guide 

 them over the vast plains, which they would have to cross in their 

 homeward journey. * * From its use on land the compass was 

 finally adapted to maritime purposes, and under the dynasty of Tsin, 

 in the fourth century of our era, Chinese vessels under the guidance 

 of the compass visited Indian ports and the eastern coast of Africa." 



The mariner's, compass was certainly known to Europeans in the 

 12th century, for in the great library at Paris is a manuscript poem 



