470 TRANSACTIONS OP THE AMERICAN INSTITUTE. 



dragged across the river Clyde. In 1769, Grimsliaw used a four- 

 spoon dredger, each spoon containing a ton of soil. Bolton and 

 Watt, in 1796, applied steam to the dredger. In 1802, Mr. Rennie 

 enlarged and improved the apparatus. In 1805, Mr. Sliorter 

 operated a spoon dredger hy the tide. In 1807, Richard Treve- 

 thick, an engineer, made further improvement in the mode of 

 dredging. In 1810, John Mills introduced machinery by which 

 50 per cent, of the applied power was shown in the results. 



At later periods the steam dredger has been much improved^ 

 and has' been constructed on a larger scale. The result has been 

 that the cost per ton has been reduced within the last twenty 

 years from thirteen pence to fovir pence. In September last an 

 iron steam dredger 110 feet in length, was constructed upon the 

 Clyde. This machine, which can operate in .30 feet of water, has 

 filled a barge of a capacity of 33 cubic yards, or 45 tons, in 15 

 minutes, in 18 feet of water. This is an efficiency about four 

 times greater than was possessed by the early machines. 



Pumping was spoken of by Mr. Johnson as being under some 

 conditions a species of dredging. In pumping, at the time of 

 constructing the government dry dock at Brooklyn, there was a 

 pound of fine sand in each gallon of water pumped out ; and 

 three thousand tons of sand were removed in that way, by the 

 Worthington pump. In an attempt of Mr. Maillefert to recover 

 quicksilver from the " Flying Dutchman,'' it was found t^at what 

 little excavation they effected in one day was filled up the next. 

 The sand in which the vessel lay was "so lively that the divers 

 sank in it knee deep. "But by the aid of the centrifugal pump of 

 W. D. Andrews, driven by an engine of 12-horse power, the mer- 

 cury and other cargo were obtained. 



Mr. Johnson then proceeded to show by experiments how 

 readily sand may be removed by pumping, either air or water, and 

 when raised by his form of pump, that it will be accompanied by a 

 comparatively small amount of water. The weight of a cubic foot 

 of dry sand is about 80 lbs. Wet to saturation it is about 100 

 lbs. A small additional quantity of water was shown to aid 

 materially in the removal of sand by his pumping apparatus. 

 Messrs. Eaton and Brother, of Charleston, S. C, with a propeller 

 dredger 150 feet in length, having a centrifugal pump six feet in 

 diameter, with a nineteen inch tube attached, raised 150 cubic 

 yards of sand, shells, &c., from a depth of twelve feet, in twenty- 

 seven minutes, and deposited the same a mile and a half distant 



