Method. 



428 



HYDROGRAPHICAL SURVEYING [chap. xx. 



and is 1 <> feet before it when the depth is 1 20 feet : the correction 

 is not, therefore, noticeable, except on a very large scale of 

 plotting. 



The roller (see Fig. 92), with both towing lines attached, is 

 hoisted out with a burton, on a slip toggle; the shij) is started 

 at slowest speed, and the roller is lowered from one lower boom 

 by its bridle, the opposite bridle being eased simultaneously, 

 until both bridles are at the marks indicating that the roller is 

 at its proper position, and are then secured permanently. 

 The depth- wire requires careful attention as the roller comes aft, 



Fig. 92. 



and the counterpoise must be put over as soon as possible, 

 in order to keep the wire taut all the time. 



When the roller is towing in place, a " leadsman " is stationed 

 to read the depths shown on the depth-wire ; and is ordered to 

 hail them at even numbers of seconds apart, as desired — about 

 every twenty to thirty seconds is a convenient distance for 

 plotting — and also when any sudden shoahng of irregularity 

 in depths is observed, in between hails. 



Note. — It will be remarked that every sounding thus taken 

 can be correctly plotted ; for the spacing between fixes for each 

 sounding will be exact. This is seldom the case in ordinary 

 sounding, when it takes considerably longer to obtain a deep 

 sounding than a shoal one. 



