8 



HYDROGRAPHY. 



The measurement of the bases being all taken within a short time of 

 each other, and the observations made by many officers at all the 

 angles of the triangles, it leaves little correction even for personal error 

 or inaccuracy in measuring the distance by sound, and as the angles 

 are simultaneously measured, there can be but little variation from the 

 truth. The objects are generally so well defined as to make their selec- 

 tion an easy task to even the most uninitiated. 



It may be thought that the movements of the vessels would be too 

 great ; but this is not of any great amount, indeed is very small during 

 the actual time engaged. There can be no doubt of the first positions 

 being occupied, when the original angles are "kept on;" besides, the 

 proof of the whole work by the bases of verification, fully assures one 

 of its accuracy as well as efficiency. Indeed, I do not know what is 

 more to be desired or obtained, except it be the rise and fall of the 

 tides, which the short time occupied does not admit. Having fully 

 tested and satisfied myself of the accuracy of the method, and the 

 celerity of execution, even by ships of any squadron, not expressly 

 fitted for surveying operations, I cannot too strongly recommend its 

 adoption whenever an opportunity offers, believing there are few things 

 that would add more to our knowledge of the changes going on in the 

 islands and shores, than the results would bring to light. 



Although we had recourse to sound, in many cases, to obtain our 

 bases on land, or rather the sides of the triangles, yet these were fre- 

 quently proven by a measured base for verification, or one obtained by 

 the subtension of a staff at a distance, where the ground did not admit 

 of its being actually measured. 



The methods which were employed in the more extended surveys of 

 the groups of islands, as well as those of the rivers, that fell to our duties, 

 require also some explanation, and for this purpose, and to illustrate 

 the methods more clearly, a diagram of each triangulation is given on 

 Plates 1 and 11. 



The Feejee Group of Islands embraces an extent of sixty thousand 

 square miles, in which are situated one hundred and eighty islands, 

 reefs, and shoals, some having areas of six hundred miles, and coasts 

 of three hundred miles, with well-sheltered and commodious harbors, 

 formed by reefs which encircle them, besides a large number of ancho- 

 rages and passages through the reefs. This group was, so far as any 

 knowledge of its hydrography and topography, entirely unknown, 

 and but few of its fine harbors had ever been visited. Its inhabi- 



