88 MEMORANDUM. 



" The circularly-formed Coral Islands in the Pacific occa- 

 sionally afford excellent land-locked harbours, with a sufficient 

 entrance, and would be well adapted to any nice astronomical 

 observations which might require to be carried on in undis- 

 turbed tranquillity. While these ai'e quietly proceeding, and 

 the chronometers rating, a very interesting inquiry might be 

 instituted respecting the formation of these coral reefs. 



" An exact geological map of the whole island should be 

 constructed, showing its form, the greatest height to which the 

 solid coral has risen, as well as that to which the fragments 

 appear to have been forced. The slope of its sides should be 

 carefully measured in different places, and particularly on the 

 external face, by a series of soundings, at very short distances 

 from each other, and carried out to the greatest possible depths, 

 at times when no tide or current can affect the perpendicularity 

 of the line. A modern and very plausible theory has been put 

 forward, that these wonderful formations, instead of ascending 

 from the bottom of the sea, have been raised from the sum- 

 mits of extinct volcanoes ; and therefore the nature of the 

 bottom at each of these soundings should be noted, and every 

 means exerted that ingenuity can devise of discovering at what 

 depth the coral formation begins, and of what materials the 

 substratum on which it rests is composed. The shape, slope, 

 and elevation of the coral knolls in the lagoon would also help 

 the investigation ; and no circumstances should be neglected 

 which can render an account of the general structure clear and 

 perspicuous. 



" A set of observations connected with the theory of the 

 tides might likewise be carried on with peculiar propriety in 

 one of these coral basins, provided the openings should be 

 sufficiently wide and deep to admit the flux and reflux without 

 material impediment. The island selected for such a purpose 

 should be nearly midway in the ocean, and not very far from 

 the equator. There the tidal wave, uninfluenced by the inter- 

 rupting barrier of one continent, and equally far from the 

 reaction of the other, might be measured with very beneficial 

 results. Delicate tide-gauges should be prepared beforehand, 



