SOUNDINGS AND HYDRAULIC SURVEYS 13 



cord and divided into feet by different colored rags or other visible 

 marks. Since the cord is liable to change its length it should be 

 compared from time to time with some standard measure. Com- 

 parative tests of the two methods of soundings were made by Col. 

 G. L. Gillespie, U. S. A. Corps of Engineers, in the harbor of New 

 York, where part of the ground had been gone over by both methods. 

 He found that the rod soundings showed a less depth than the line 

 soundings by an average of six inches. In these experiments a 

 14-lb. lead was used and the line was compared at frequent intervals 

 with a steel tape in order to verify its accuracy and when not in 

 use was kept lying in fresh sea water. 



(c) To make soundings by any one of the two methods just 

 explained requires some length of time; to proceed more rapidly 

 automatic soundings were devised. These consist either of a grad- 

 uated rod or a sweep. In both cases the lower end of the apparatus 

 is kept resting on the bottom, while the depth of the various points 

 is recorded automatically. 



On .the Delaware River at Philadelphia the soundings were 

 made from a long raft by means of a weighted wooden sounding 

 rod, the raft being shifted transversely to its length, for successive 

 rows of soundings, by long anchor cables. The method of the sweep 

 was found of more practical application. Thus in the same work 

 at Philadelphia was used a short horizontal sweep which was passed 

 over the bottom; the sweep was either held by vertical gauge rods 

 attached to a boat which was rowed over the ranges, or it was manipu- 

 lated by a diver working from a scow in tow of a tug. 



Based on the same principle of a sweep bar, a new device for 

 recording automatically the various soundings was constiucted 

 by Mr. R. M. Pardessus and described in the Eng. News, Vol. XLIX. 

 The apparatus is carried by a barge which is moved along fixed 

 ranges in tow of a tug at uniform speed. The sounding is made 

 by a transverse sweep bar fastened to the outer ends of two arms, 

 which are pivoted one on each side of the barge and extend rear- 

 wardly down into the water. The sweep is made of a piece of rail- 

 road rail to give the necessary weight. At the upper end of the 

 hinged arms is a recording apparatus which marks the profile of 

 the bottom on a strip of paper moved by clock-work. The speed 

 of the tug and the motion of the paper being adjusted to a known 

 ratio, the curve traced on paper is a scale profile of the bottom 

 along the range over which the sounding barge is towed. 



