OCEANIC DISCOYEKIES. 669 



In consequence of the precession of the equinoxes, the 

 starry heavens are continually changing their aspect from every 

 portion of the earth's surface. The early races of mankind 

 beheld in the far north the glorious constellation of our 

 southern hemisphere rise before them, which, after remaining 

 long invisible, will again appear in those latitudes after the 

 lapse of thousands of years. Canopus was fully 1 20' be- 

 low the horizon at Toledo (39 54' north latitude), in the 

 time of Columbus ; and now the same star is almost as much 

 above the horizon at Cadiz. While at Berlin, and in the 

 northern latitudes the stars of the Southern Cross, as well as 

 a and /3 Centauri, are receding more and more from view, 

 the Magellanic clouds are slowly approaching our latitudes. 

 Canopus was at its greatest northern approximation during 

 the last century, and is now moving nearer and nearer to the 

 south, although very slowly, owing to its vicinity to the 

 south pole of the ecliptic. The Southern Cross began to 

 become invisible in 52 30' north latitude 2900 years before 

 our era, since, according to Galle, this constellation might 

 previously have reached an altitude of more than 10. When 

 it disappeared from the horizon of the countries on the Baltic, 

 the great pyramid of Cheops had already been erected more 

 than five hundred years. The pastoral tribe of the Hyksos 

 made their incursion seven hundred years earlier. The past 

 seems to be visibly nearer to us when we connect its measure- 

 ment with great and memorable events. 



The progress made in nautical astronomy, that is to say, 

 in the improvement of methods of determining the ship's 

 place (its geographical latitude and longitude), was simultane- 



marks the hour each day 3' 56" earlier. I am indebted to the commu- 

 nications of my friend, Dr. Galle, by whom Le Terrier's planet was first 

 discovered in the heavens, for all the calculations respecting the visi- 

 bility of southern stars in northern latitudes. " The inaccuracy of 

 the calculation, according to which the star a of the Southern Cross, 

 taking refraction into account, would appear to have begun to be invi- 

 sible in 52 25' north latitude, about the year 2900 before the Christian 

 era, may perhaps amount to more than 100 years, and could not 

 be altogether set aside, even by the strictest mode of calculation, as 

 the proper motion of the fixed stars is probably not uniform for such 

 long intervals of time. The proper motion of a Crucis is about one-third 

 of a second annually, chiefly in right ascension. It may be presumed 

 that the uncertainty produced by neglecting this, does not exceed the 

 above-mentioned limit." 



