434 



HYDROGRAPHICAL SURVEYING [chap. xx. 



The error, however, rapidly increases beyond that Hmit; so 

 that it would be necessary to wait for slack Avater of tidal 

 streams, or better weather, in the case of wind, should a larger 

 angle be necessary to keep the ship on the line desired to be 

 run. These conditions, of course, vary with every ship, accord- 

 ing to the lengths of her towing fittings, as well as to the depth 

 in which the roller may be ; in the latter case, the error is less 

 in greater depths, and more in lesser depths. 



Fig. 94. 



It may be remarked that a fitting of this nature could prob- 

 ably be adapted for boats; for example, by a steam cutter 

 towing a roller or sledge along the bottom, at a convenient 

 distance, Avith a small boat perpendicularly over it, carrying 

 the depth-line, and the officers jQxing the position. The read- 

 ings might even be automatically recorded, as in a tide-gauge. 

 In the case of a boat, however, a towing bridle would have to be 

 dispensed with, and a straight course carefully preserved by a 

 transit, or otherwise. 



D0UGLA8-SCHAFER SQUNDING TRAVELLER. 



This is a contrivance devised to obtain soundings rapidly from 

 a ship under way, and is the result of the ingenuity of Com- 

 mander H. P. Douglas, R.N., and Lieutenant J. 8. Schafer,R.N. 



Continuous soundings have been obtained with it, in depths 

 of from 20 to 40 fathoms, at an average rate of one sounding 

 per minute. 



It is used in conjunction with a Avu-e jackstay running from 

 the head of the sounding spar to the end of the loAver boom 

 and is s-uppHed to all H.M. surveying vessels. The following 



