ADMIRALTV INSTRUCTIONS. 21 



nautical, will suggest themselves to your active 

 iiiiiid ; and though, from your transient stay at any 

 one place, you will often experience the mortifica- 

 tion of leaving them incomplete, yet that should 

 not discourage you in the collection of every useful 

 fact within your reach. Your example in this 

 respect will stimulate the efforts of the younger 

 officers under your command, and through tliem 

 may even have a beneficial influence on the future 

 character of the navy. 



"It has been suggested by some geologists, that 

 the coral insect, instead of raising its superstruc- 

 ture directly from the bottom of the sea, works only 

 on the summits of submarine mountains, which 

 have been projected upwards by volcanic action. 

 They account, therefore, for the basin-like form so 

 generally observed in coral islands, by supposing 

 that they insist on the circular lip of extinct 

 volcanic craters ; and as much of your work will 

 lie among islands and cays of coral formation, you 

 should collect every fact which can throw any light 

 on the subject. 



" Hitherto it has been made a part of the duty 

 of all the surveying vessels to keep an exact register 

 of the height of the barometer, at its two maxima 

 of 9, and its two minima of 3 o'clock, as well as 

 that of the thermometer at the above periods, and 

 at its own day and night maximum and minimum, 

 as well as the continual comparative temperature of 

 the sea and air. This was done with the view of 



