GEOGRAPHY. 143 



Coral Islands. 



The coral-making animals do not commence their 

 labours at the extreme depth of the ocean, but on 

 rocky shoals, the summits of submarine mountains, 

 round which they form a united chain, irregular in 

 shape, but generally approaching more or less to a 

 circle. The outer ledge of the reef exposed to the 

 surf of the sea, is the first that shows itself above 

 water; in process of time it becomes indurated, 

 breaks and crumbles by the action of the sea, and at 

 length forms a barrier, within the sloping sides of 

 which the living animals are seen carrying on their 

 operations. Those observed by Chamiso (the natu- 

 ralist with Lieut. Kotzebue,) were the tubipora mu- 

 sica, millepora, cselurea, obstichopora, and various 

 kinds of polypi. As soon as the ledge has reached 

 such a height that it remains almost dry at low-water, 

 the coral insects leave off building any higher. 



Coral reefs rise almost perpendicularly on the 

 windward side, sometimes from the depth of 200 

 fathoms. 



Two Latitudes. 



Strictly speaking, we have now two latitudes to 

 the same place ; one that determined by astronomi- 

 cal observation at the surface of the earth, and the 

 other the latitude corrected for the earth's ellipticity, 

 and known by the appropriate name of Geocentric. 



Mercator's Chart. 



In Mercator's Chart, or projection, there are none 

 but right lines ; all the meridians are equidistant ; 

 but in order to obtain the true bearing by the com- 

 pass, the spaces between the parallels of latitude are 

 made to increase as they recede from the equator, so 

 that in high latitudes they become prodigiously great. 



The peculiar advantages of this projection are, 



