HYDROGRAPHY. 517 



speed of the vessel will permit, and at reasonable distances the deep- 

 sea lead should be employed to obtain actual depths. Positive sound- 

 ings exceedinar 100 fathoms should be obtained as far to seaward as 

 circumstances will permit the survey to be extended. 



A difficult task of the hydrographic surveyor is, to search for the 

 islands and dangers shown on tlie charts, or enumerated in nautical 

 guides as uncertain in position or of doubtful existence. 



Many facts show that the origin of a gi-eat number of these may 

 be traced to deceptive appearances, to misplacement from faulty ob- 

 servations or reckoning, or to typographical errors in the reports 

 published. 



Reports of new dangers grow more frequent, as the sea-routes 

 extend into regions hex-etofore but little traversed, and as the com- 

 mercial navigator manifests a greater interest in hydrography. All 

 these obstructions to navigation are placed on the charts, usually with 

 queries, until they are verified and correctly located, or their non- 

 existence proved by professional authority through local search. Such 

 dangers have frequently been found to exist at considerable distance 

 from the positions given, from indifferent astronomical observations, 

 or from reckoning referred to observations taken several days before 

 or after their discovery ; the search must, therefore, be extended over 

 a considerable area. The search for islands is naturally less difficult 

 than that for submerged dangers, which on the broad ocean can in 

 some instances hardly be detected but by chance. 



In causing reported dangers of this nature to be erased from the 

 charts, on the strength of a search which has not been thorough 

 in every particular, the hydrographer incurs a grave responsibility; 

 there are a number of instances on record where dangers which 

 had been searched for most carefully and by very competent authority, 

 have been replaced exactly in the position from which they were 

 erased, after they have been assured by the loss of a vessel on them, 

 and the reexamination of the position in consequence of it. 



A correct representation of the character of the bottom of the 

 waters is very important, not only for the selection of anchorages, 

 but also as a guide to the navigator when he cannot otherwise obtain 

 the position. of his vessel, especially when approaching a coast in fogs 

 and thick weather, or when passing through channels not bordered 

 by good landmarks ; for this purpose specimens of the bottom should 

 be brought up for examination, and every change of it noted. 



The tidal relations, tidal hour, and the rise and fall at the various 

 stages of the moon, and in the various seasons, the influence of the 

 winds upon the tides, etc., can be deduced accurately only by obser- 

 vations continued through a longer period than the limited time of a 

 running survey will generally permit. Observers should, if possible, 

 be left for this purpose at the important points. A lunation is the 

 shortest period in which approximate data can be arrived at, but ob- 



