CHAP. XX.] ROLLER SOUNDING 429 



If the ship's course has been straight between fixes, it may 

 be said that each roller-sounding is accurately fixed, by time, 

 and not merely those at which sextant angles have been taken. 

 This is a great advantage on large-scale work; and the result 

 is very apparent where fines of soundings cross one another; 

 these rarely showing any discrepancies. 



The roller is of the form of an ordinary garden-roller, in two Descrip- 

 halves, to permit of easy turning when altering course. It is p^^inL 

 made of mild steel, the ends of both sections being covered, (see 

 and the interior fiUed with lead to bring it to a weight of *^' ^^^• 

 500 pounds, which is found to be a convenient size for manipula- Roller, 

 tion. It requires to be strongly constructed to stand the roUing 

 over the sea bottom, especially when unexpected rocky ground 

 comes in its path. The towing-lines are shackled directly to 

 the eye in the end of the to wing-handle, and the depth-fine 

 to the eye of the tripper. 



The main objection to the system of towing the depth-in- Tripper, 

 dicator to obtain soundings is the difficulty of knowing whether 

 or not the object towed — in this case the roller— is actually 

 on the bottom. This has been overcome by a fitting, consisting 

 of a tripper arrangement on the roller, to which the depth-wire 

 is attached, and of a series of ribs, or battens, riveted to the 

 surface of the roller, arranged so that, as the roller revolves, the 

 toes of the tripper, passing over the battens, which are of tri- 

 angular section, give a series of small jerks to the depth-wire, 

 which shall be observable above the water. It has been found 

 in practice that these jerks are scarcely ever observable to sight, 

 because the irregularities of the bottom cause greater jerks 

 to the line than those caused by the battens on the tripper, thus 

 rendering the latter imperceptible; but it has also been found 

 that they sometimes can be distinguished by feeling the wire, 

 and, more certainly than in either of these methods, if the ear 

 be placed against the foot of the spar which carries the depth- 

 line block, the continuous and even rattling of the tripper pass- 

 ing successively over the battens on each section of the roller 

 can easily be distinguished — the sound ceasing directly the 

 speed of the ship is increased sufficiently for the roller to tow 

 clear of the bottom, and thus to cease revolving. 



These are lengths of 1-inch steel-wire rope, and require no Towing- 

 fitting, beyond a thimble turned into the end for shackling to ^^°®^- 



