CHAP. VIII.] SHIP SOUNDING 211 



of keeping the lead-line clear of all projections on the ship's 

 side. 



The lower boom being got out and drooped as far as the 

 rolling of the ship will permit without dipping it, a wire 

 jackstay connecting the end of the boom with the sounding 

 davit, is set taut by means of the foreguy. 



The slip holding the lead is attached to a carrier travelling 

 on rollers on the wire jackstay, and is hauled forward by the 

 rounding-line. The lead is slipped by the tripping-line, which 

 also hauls the slip back again to the sounding-davit in readiness 

 for the next sounding. The wire jackstay being inclined 

 downwards, the lead runs forward freely if the jackstay is 

 kept taut. 



Fig. 58 shows the arrangement, which has been found to 

 work satisfactorily, and is more expeditious than the former plan. 



A variety of instruments has been invented for giving instru- 

 the accurate depth when the line cannot be got up and down ; ^goJ^ing 

 some depending on a fan which works a series of cogged Depth, 

 wheels, as Massey's ; others on pressure at different depths. 

 These are all useful up to a certain point, and when their 

 errors have been obtained, may sometimes be attached to the 

 lead with advantage. 



Recorders, however, of great value to navigation are of 

 no use in surveying operations, and the majority of these 

 navigational inventions are liable to small errors, which we 

 must not have in depths which are to be placed on charts. 



Burt's bag and nipper are useful when the ship drifts away 

 from the vertical position over the lead, and one should always 

 be handy when sounding, but great care is necessary that it 

 bites the line. 



It is evident that a 'perfect machine is more trustworthy Perfect 

 than the record of an up-and-down cast with the ship in JI^^^JjJ^® 

 motion, as given by a fallible man ; and when such perfect Made, 

 machine is invented it will be gladly adopted by surveyors ; 

 but up to the present time the machines are more liable to 

 error than a trained man under most circumstances. 



A wire sounding machine on the poop is very useful for Wire 

 depths over 60 fathoms. Under that depth the hemp ^°"chtn? 

 sounding-line is more expeditious. Using an ordinary 28- on the 

 pound deep-sea lead connected with the end of the wire °°^" 



14—2 



