Hydrography^ and the Art of Navigation. 87 



tion derived from the immensity of the space in which the phy- 

 sical changes must operate, it has lost all its force since the phe- 

 nomena of the same nature exhibited by Halley's comet. 



Our young countrymen ought therefore zealously to devote 

 themselves to the observations which we have pointed out. The 

 question is important, and no one can hitherto flatter himself 

 with having definitively solved it. 



Aurora Borealis. — It is now well ascertained that there are as 

 many displays of polar aurora towards the southern hemisphere, 

 as in the Arctic Regions. Every thing leads us to think, that the 

 appearances of the southern aurora, and of that which we wit- 

 ness in Europe, follow the same laws. This, however, is mere 

 conjecture. If a southern aurora be seen by the officers of the 

 Bonite in the form of an arch, it will be important to notify ex- 

 actly the orientations of the points of this arc's intersection with 

 the horizon, and, if these cannot be obtained, the orientation of 

 the most elevated point. In Europe, the most elevated point 

 alwaj's appears to be situated in the magnetic meridian of the 

 place where the observer is stationed. 



It has been proved by numerous researches undertaken at 

 Paris, that all kinds of aurora boreahs, even such as do not 

 appear above our horizon, and the existence of which, conse- 

 quently, we can learn only from the reports of observers in the 

 polar regions, alter decidedly the declination of the magnetic 

 needle, as well as its inclination and intensity. Who, then, can 

 presume to assert, as an argument to prove the great distance 

 of the aurora borealis, that it never disturbs the magnetism of 

 our hemisphere ? The attention which our travellers, in every 

 case, bestow on these phenomena, may at length throw some 

 light on the question. Arrangements have been formed, in order 

 that magnetic observations may be made at Paris during the 

 whole time of the Bonite's absence, at periods so near each other, 

 and in such a manner, that no irregularity can take place un- 

 perceived. 



The Rainhow. — The explanation of the rainbow may be re- 

 garded as one of Descartes's most beautiful discoveries ; but, at 

 the same time, even after receiving the developments which 

 Newton gave to it, it is still incomplete. When we look atten- 

 tively at this magnificent phenomenon, we perceive under the 



