STIRRING DREDGES 143 



$20,000 per each steamer, but since the relief was only temporary 

 and had to be repeated from year to year, it finally gave place to 

 the so-called permanent improvement, consisting mainly of channel 

 contraction. 



Stirring by Revolving Wheels. The agitation of the bottom by 

 means of revolving wheels has been done in two different ways, viz., 

 by the Bishop conical screws and by propellers. 



The Bishop conical screws consist of two conical screws abutting 

 together at their vertices and spreading out at their bases so as to 

 form a flat letter V. These are supported by a heavy upright frame 

 to be attached to the bow of a vessel and arranged to raised or 

 lowered so as to bring the screws in contact with the bar, the point 

 of the V being forward. The screws are revolved on bearings at 

 the two ends of each by guard wheels, run by the engines of the 

 vessel to which the machine is attached, or by an auxiliary engine 

 if desired. They are connected with the motor by endless chains. 

 The screws, revolving in opposite directions, were expected to cut 

 a furrow through the bar, breaking up its surface and washing it up 

 into the river current to be floated away. This device was applied 

 to the steamer "Wiggins " in 1867, but gave unsatisfactory results 

 owing to the fact that the screws, lowered to 16 or 18 ft. into the 

 water and projecting far from the front of the vessel, made the 

 machine clumsy, so it became unmanageable in the muddy water. 



The steamer "Wiggins," on which the Bishop screws were tried, 

 was provided with two conical screws 20 ft. long, 5 ft. in diameter 

 at their bases, and placed in such a manner that their points came 

 together in front of the boat's cutwater, and their bases were sep- 

 arated from each other so as to measure about 20 ft. from "out to 

 out. " They were mounted so that their axes were horizontal. 

 Their flanges were 12 ft. wide at the base of the cones, diminishing 

 to 6 in. at the points. 



Propellers. Propellers of large diameter were also used to stir up 

 the materials from the bottom of channels. In the year 1859 Mr. 

 Charles Hyde, noticing the small progress made by the scrapers in 

 removing sand-bars, decided to adopt more powerful means of 

 agitating the materials and used the propeller " Enoch Train." A 

 vessel was constructed with two ship's propellers at the stern and 

 with water tanks which could be filled to sink the hull so as to bring 

 the propellers in contact with the bar and by their revolution cut 

 and stir it up, to be swept aw r ay by the river current. 



