CHAP. XX.] ROLLER SOUNDING 433 



floor, no more certain method can be adopted. When the 

 roller is towing, it is most interesting to see on the depth-line 

 how every small irregularity of the bottom is exhibited, as 

 well as the larger and more important elevations and depres- 

 sions. The Tarbert Bank, off the west coast of Jm'a, was 

 almost entirely sounded, and its 10-fathom limit line delineated 

 by this method, the whole of the roller fittings (with the ex- 

 ception of the two actual sections of the roller) having been 

 made on board by the ship's blacksmith. 



As regards the question of speed, it is found that a 500-pound 

 weight, as described, leaves the bottom at 15 fathoms directly 

 the speed is increased above 3| knots ; and in 20 fathoms depth, 

 the speed must not be much above 2| knots. It is considered 

 that, on account of sag, the speed should not, even in 5 fathoms 

 depth, be above 4 knots, though the roller still remains on the 

 bottom in those conditions. 



As regards the question of a tidal stream or wind acting on 

 the ship in any other direction than right ahead, or right astern, 

 so that she has to have her head directed at an angle to the 

 Une it is desired to run over the ground, it will be noticed that 

 this angle may be so great that the roller will assume a j)osition 

 at some distance off on the quarter, instead of dead astern; 

 and that, consequently, the depth-line, coming to the surface 

 at a greater angle than calculated for the astern position, will 

 give an incorrect reading of the dejDth. 



The size of the angle to which the ship may thus be laid off 

 her course-made-good, without error of depth-line reading, 

 is governed by the proportion which the distance between the 

 blocks on the boom-ends, through which the towing bridles 

 lead, bears to the length, calculated along the centre Une of the 

 ship, at which the roller is towing. 



In Fig. 94, if R be the position of the roller, B the towing 

 block on the boom-end, AR the centre line of the ship, then 

 the roller will tow dead astern so long as the ship is not off her 

 course-made-good at a greater angle than ARB. 



In the Research, for example, this angle, when the roller 

 is at a depth of 50 feet, is 12° 20'; but as a further 10° off her 

 course only makes an incorrectness in the depth-line reading of 

 3 inches, it may be said that the ship may be about 25° off her 

 course-made-good without sensible error in the soundings as read. 



28 



