a6 THE IMPROVEMEXT OF RIVERS. 



and contract daily. Desired distances are marked by tags made of metal or of leather, 

 and attached by light wires or twine to the cord or wire. The depth of the water at 

 these tags may then be found by the use of a graduated pole or a sounding-lead manip- 

 ulated from a rowboat. The pole should be heavy at the bottom, with a flat end, 

 so as to give the depth accurately without sinking into the material of the bed. In 

 deep water and swift currents it will be necessary to resort to the lead. This is a heavy, 

 slender weight, attached to a chain or rope which has been stretched and graduated. 

 If a rope is used, tests of the line should be made often and its variations noted, in order 

 that proper corrections can be made in reducing the notes. The usual method of ob- 

 taining the depths is to permit the sounding-boat to float down stream and let the lead 

 strike bottom just as the range-line is crossed. If a steamboat is employed, the line is 

 stretched along the guards with the lead at the bow, from which point it is dropped into 

 the water and the reading is taken at the stern when the line becomes perpendicular. 



Another method is to row the boat across the river along the range-line. The 

 leadsman occupies the bow of the boat which is kept well up under the wire, and near 

 him is the recorder, who makes a note of the stations and the depths called out by the 

 leadsman. 



By Range-line and Instrument. In wide streams a range-line is established and 

 defined by two numbered targets on the shore, and the sounding or depth stations are 

 fixed by angles taken by a transit placed on a base-line on the bank, or by a sextant 

 on the boat. The details of the method are as follows: * 



"For cross-section soundings all taken are 'drifting soundings.' The boat is run 

 up above the cross-section line being sounded, the lead cast and held about one foot 

 from the bottom. As the boat drifts past the range-line, the lead is allowed to touch 

 the bottom, the depth noted and recorded. 



" In shallow water the soundings are taken from a skiff, a steam-launch being 

 employed in the deep water and rapid currents. 



" Each cross-section is marked by two range-signals. These are made either of 

 one-inch boards about three feet square, painted white, or of white cotton, fastened 

 to a frame of about the same size. These are attached to a post about 1 2 feet in length, 

 securely planted in the ground, or to a small tree, the front signal being placed near 

 the river-bank, and the rear signal from 500 to 1500 feet inshore. This arrangement 

 clearly defines the range-line at any point in the river, and enables the leadsmen to 

 determine when the boat is exactly on range, so that soundings can be obtained on line, 

 and renders unnecessary the use of more than one instrument for locating the soundings. 



"The location of all soundings taken is determined instrumen tally as follows: 



"Those taken from the steam-launch are located by an observer on board with a 

 sextant, by one angle to two of the stations on shore. When soundings are taken from 

 a skiff, an observer with a transit occupies one of the stations on shore, at a sufficient 



* Annual Report, Chief of Engineers, U. S. A., 1883, pp. 2192 and aaio. 



