CHAP. XX.] ROLLER SOUNDING 425 



ROLLER SOUNDING. 



By captain B. T. SOMERVILLE, R.N., H.M. .Surveying Ship 



'■ Research." 



This method of obtaining .soundings, by which the depth is 

 continuously recorded, has been found to be a vahiable one so 

 long as its natural limitations are not exceeded. These include: 



1. A speed of shij) not exceeding 3| knots. 



2. A depth not exceeding 20 fathoms. 



3. A smooth sand or mud bottom. 



4. A tidal stream, or wind of such a force that the ship will 

 not have her head dii'ected more than about 25° off the course 

 she is making good. 



It would prove most valuable, for example, in the sounding 

 of banks in river estuaries, or on the East Coast of England; 

 and provided the bottom was sufficiently smooth, and the tidal 

 stream and weather suitable, would be ideal for sectional 

 sounding work. 



A roller, of the form of an ordinary garden roller, is towed General 

 along on the bottom by a bridle consisting of two wire ropes, tion^fsee 

 coming from blocks near the ends of the two lower booms. Fig- soj. 



A marked depth-line, of fine wire, is attached to the roller, 

 and comes from there to the surface, being rove through a block 

 at the head of a spar over the counter of the ship, and kept 

 taut by means of a counterpoise weight. 



The bridle is arranged of such a length that the roller shall 

 be perpendicularly beneath the head of the sounding spar aft 

 when the depth is 100 feet. As the towing lines are secured at 

 this permanent length, it follows that for all depths less than 

 100 feet the roller will be abaft the perpendicular, and for all 

 depths over 100 feet will be before it. 



The depth-line is, therefore, at a certain angle from the 

 perpendicular for each particular depth other than 100 feet; 

 and a calculation is thus necessary to find the position for the 

 marks on the depth-line so that it may show the perpendicular 

 depth over the roller. Fig. 91 shows the problem of the 

 calculation, which, as will be seen, is a simple one. It is 

 readily solved by protraction from calculating paper, with the 

 conditions drawn on it to scale, 



It will be observed that it is better to have the blocks on 



