CHAP. XX.] ROLLER SOUNDING 431 



the roller to wing-handle. They require to be carefully 

 measured, and to have marks tucked into them, to show 

 when the roller is towing at the required position, according to 

 the calculations derived from the ship's measurements. 



It is advisable to have two marks in each line; one to be at 

 the block on the boom end, the other at some position in the 

 ship {e.g., the fairlead in the knightheads), so that it may be 

 seen that the boom is itself at its proper altitude to suit the 

 calculated lengths. 



Since the marldng of the depth -wire depends on everything 

 being at the exact positions in which they were measured for 

 the original calculations, it is essential that these towing-hne 

 marks shall be carefully adhered to. The boom guys and 

 topping lifts should be marked similarly. 



This is made of |-inch special flexible steel-wire, and is Depth- 

 marked as a lead-line in feet, according to the calculated posi- ^^^' 

 tions for the marks; so that (for example) when the 40-foot 

 mark is at the surface, it indicates that there is 40 feet perpen- 

 dicular depth over the roller — ^all depths, of course, being 

 measured from the bottom of the roller. 



A thimble is turned into the lower end for shackling to the 

 tripper on the roller; and the wire is then rove through the 

 block at the head of the sounding spar, then through the block 

 on the counterpoise, and the end brought inboard. 



As it is necessary to alter the position of the counterpoise 

 as the depths alter, to prevent it touching the bottom, and as 

 the depth-wire is of too small a size for proper manipulation 

 by hand, a convenient disposition of the inboard end has been 

 found in reeving it, after it has passed through the counterpoise 

 block, through the block on the submarine sentry davit, and 

 securing it to the drum of the sentry winch. It can then be 

 eased out, or hauled in on this drum, as required without diffi- 

 culty or loss of time, by the winch-handle. 



A 11 2-pound lead is used for this, with a 2^-inch metal snatch Counter- 

 block (surveying pattern) secured to the ring at the top for P°*^®- 

 the depth-Hne. It is advisable to have a preventer line made 

 fast to the counterpoise, so that it shall not be lost in case of 

 the depth-line carrying away. Should the roller encounter a 

 rocky patch on the bottom, the sudden rises and falls that take 

 place when passing over it naturally cause equally sudden falls 



