HYDROGRAPHIC SURVEYS. 27 



distance above or below the range that is being sounded to give good intersections. 

 The skiff is rowed a sufficient distance above the line being sounded to allow the lead 

 being cast and held near the bottom, and the boat to drift along with the current, so 

 that as the range is crossed the lead -line is as nearly vertical as possible; at the moment 

 of crossing the range the recorder calls 'Sound! ' and raises to a vertical position a small 

 flag which he holds in his hand as a signal to the observer on shore. The position of 

 the lead-line at the time of sounding is located by an angle taken from shore. The 

 depth of water and the nature of the bottom is entered by the recorder, and the boat 

 is rowed to a new position. The cross-section soundings thus obtained are generally 

 from 50 to 100 feet apart. 



''The lead used is about 22 pounds in weight, slightly hollowed out at the bottom, 

 the space being filled with tallow so as to obtain specimens of the bottom at every cast 

 of the lead. The line is an ordinary \- or J-inch cotton line, or one of Italian hemp or 

 sea-grass, well stretched and marked with leather tags at every foot ; it must be fre- 

 quently tested. These tests should be made from day to day, and when the length 

 of the line has changed a sufficient amount to affect the accuracy of the work the line 

 must be retagged. A swivel attached to the line near the lead will prevent the line 

 from twisting when in use. 



" Longitudinal soundings are taken from a skiff. The range across the river is 

 divided up into about equal distances so as to allow ten lines to be taken in the width 

 of. the river. The boat is located in position by the aid of range-signals on 'shore. An 

 auxiliary range is placed about 100 feet above the upper range, so that the boat can be 

 rowed to the intersection of the two ranges, the lead-line run out, and everything be 

 in readiness to begin sounding as the boat drifts past the upper range. By this time 

 the boat has lost its momentum, due to rowing, and is drifting along naturally with 

 the current. 



"By Two Transits. For cross-section soundings the position of each may also 

 be determined by intersections from two transits, one at each end of the base corre- 

 sponding to the ranges to be sounded. The ranges are permanently marked by stakes 

 which have holes bored vertically in their tops to receive movable flags. The range 

 to be sounded is designated by two flags on each side of the river, one being planted 

 in the permanent stake on the levee, and the other in the ground at the water's edge, 

 and carefully lined on the range. These inner flags are always located by the instru- 

 ments, and their positions recorded as those of the first and last soundings, each with 

 a depth zero. 



"Soundings are taken from a launch or from a skiff, the party consisting, in addi- 

 tion to the crew, of a rodman in charge, who acts as a recorder of the soundings, and 

 a leadsman. The boat is signaled up and down stream by a flagman stationed at 

 the shore flag; he directing the boat a sufficient distance above the line, so that it would 

 acquire, by drifting back, such velocity as would prevent the lead-line from sagging. 

 The leadsman is stationed in the bow of the boat, where there is a mast or staff with 



