432 HYDROGRAPHICAL SURVEYING [chap. xx. 



and rises of the counterpoise. These falls have occasionally 

 been found sufficient to cause the dei)th-wire to part, and re- 

 quire guarding against in arranging a preventer line. 



To employ a heavier depth-line is not recommended on account 

 of the sag that would be occasioned by its greater surface of re- 

 sistance; nor is the accident sufficiently common to warrant it. 

 Lower The ship's ordinary lower booms have been found equal to 



™^' the work entailed on them as spreaders of the towing-lines. 



The blocks for taldng the wu-es are 10|-inch iron blocks 

 (pattern 474a), shackled to the band for the guys and topping 

 lifts. 



The fore-guy should be rove double for greater support. 

 The booms should be dropped to the height above the water 

 required by the calculations (in Research, 3 feet), and both 

 guys and topping hft marked for this position. 

 Remarks. This method of sounding has been used considerably, in 

 suitable places, during the last two seasons in H.M.S. Research. 

 Its readings have constantlj^ been checked by soundings taken 

 in the ordinary manner, and have always proved to be exafttly 

 the same in all depths uj) to about 20 fathoms. 



It was fii'st carried out by means of a '" sledge," in place of 

 a roller, also weighing 500 pounds; a roller was substituted as, 

 being in two sections on the same axle, it follows more satis- 

 factorily the path of the ship, according to the alterations of 

 helm, or to the sagging due to tide, or wind; also, it tows more 

 lightly than the sledge. Perfectly good results are, however, 

 obtainable with the latter ; and it has the merit of being easily 

 constructed on board the ship, which a roller scarcely could be. 



The necessary gear is easy to rig and unrig, and has hitherto 

 stood well all the wear and tear of various descriptions of sea 

 bottom, without mishap (except once or twice to the depth-Hne, 

 as mentioned above), including unexpected patches of rock. 



It is not considered, however, that it should, generally 

 speaking, be employed on known rocky or coral grounds, as 

 the strain brought on the booms, and the probability of the 

 bight of the towing bridle catching in a pinnacle or coral 

 boulder, would almost certainly bring about an accident. 



The best resvilts are obtained when the depths are betAveen 

 30 and 80 feet, and if it is desired carefully to dehneate the 

 upward swing of a bank of sand or mud, from a level ocean 



