12 



HYDROGRAPHY. 



A few remarks are also necessary respecting the search made by 

 the Expedition for the numerous reported shoals or vigias, embraced 

 in my instructions and published in Congressional documents. The 

 many positions assigned these shoals, from their situations being often 

 determined by meridian distances from places whose longitudes are 

 not well established, renders the search for them by one vessel ex- 

 tremely uncertain ; indeed, the ineffectual search and various positions 

 assigned these doubtful islands and shoals have increased the confusion, 

 — the only result that can reasonably be expected from the imperfect 

 manner in which the examinations have been made. It would save 

 much time and trouble, and much of the difficulty would be avoided, 

 if, when an island or shoal is discovered, the navigator would state the 

 longitude of the place where the error of the chronometers had been 

 ascertained, and the meridian distance from it. Any one at all ac- 

 quainted with navigation is aware how doubtful the position of a vessel 

 becomes, even after the lapse of a few hours from the last observations, 

 however vigilant the officers may be. It is this which renders the 

 search by a vessel of so little value ; in fact, very few are warranted in 

 the assertion, that the part of the ocean in question has actually been 

 examined, and is free from danger. At a short distance from the ves- 

 sel, the very danger sought for, may exist, and still remain unseen. 

 But this can hardly occur in a search made by a squadron, which in- 

 sures the utmost certainty, and can be performed in much less time. 

 Aware of the discrepancies existing in the determination of the posi- 

 tions, creating a doubt as to their actual place, and that parts of the ocean 

 have been examined where no dangers whatever existed, a mode of 

 search was adopted to effectually guard against these mistakes, or to 

 cover and stretch over a sufficient space to make full allowance for 

 them. The longitude was considered as most doubtful, and liable to 

 the greatest errors. Errors in latitude are of less amount ; indeed, 

 there are but few observers who are not able to determine the position 

 with respect to latitude correctly. The errors which exist are for the 

 most part occasioned by the position being deduced from the dead 

 reckoning ; but even this is seldom? very remote from the truth in 

 latitude. Taking these things into consideration, a course was pursued 

 that has put to rest the question of the existence or non-existence of 

 these islands, shoals, &c. On arriving at the parallel of the island, 

 reef, or shoal, such a position was sought as would enable us to begin 

 the search, either to the eastward or westward, as the prevailing winds 



