44 THE IMPROVEMENT OF RIVERS. 



Jets. In the various inventions brought forth the water-jet has had a prominent 

 place, the object being to enable a steamboat, upon running onto a bar, to work its 

 way through by means of pumps, and some of these devices have met with considerable 

 success. In 1881, for example, a bar near St. Louis was cut through by the use of 

 pumps mounted on boats, and having a capacity of about 165 gallons each per minute. 

 In ten hours the channel was deepened from 6 feet to 8.3 feet, and made wide enough 

 for the largest tows. 



A special jet-dredge was built in 1896, for use on the Mississippi between St. Louis 

 and Cairo, the pumps being two 1 5-inch centrifugals, each with a capacity of 10,000 

 gallons per minute. On short bars the work was very effective, but on long ones the 

 piling-up of the sand in front of the jets prevented successful results. 



Dredging. Dredging as a means of river improvement has been extensively 

 resorted to in all parts of the world. It is in the harbors and mouths of streams, 

 however, that it has had its widest application. With the exception of the lower 

 Mississippi, where it is in use as a means of giving temporary relief during low-water 

 seasons, its employment in this country has not been on an extended scale on non-tidal 

 streams. In connection with blasting it has been found quite efficacious on some 

 of our best rivers, and where the river-bed is of a fairly hard material the results from 

 dredging have been generally satisfactory. However, in the majority of cases, redredg- 

 ing has been found necessary from time to time in order to maintain the desired depth, 

 this being the great objection to this process of improvement. When the material 

 is taken out it must have a place of permanent deposit, or it will be washed into chutes 

 lower down stream, and the best results from dredging are only attained in those 

 streams and those places where there is little drifting material. A bar at the mouth 

 of a creek may be removed each year, and still navigation will be impeded by new 

 material coming into the cut excavated. This process of improving navigation is not 

 confined to open rivers, but is carried on to a considerable extent in those streams 

 having works of canalization. 



Types of Dredges. The following are the principal types of dredges in use: 



Dipper Dredge. This consists of a boat provided with an iron dipper or bucket, 

 secured to the end of a long handle, which is mounted on a revolving boom so as to 

 permit a wide range of digging. To operate it the dipper is lowered on to the bed of 

 the river, and pulled along in an almost horizontal direction till it has scraped up a 

 load. It is then raised (the handle being arranged to slide up and down and held 

 wherever desired by friction), the boom is swung, and the load dumped into scows 

 or elsewhere by opening the hinged bottom of the dipper. The boat is held in position 

 by "spuds," or long timbers, which move vertically in sockets in the hull, and are let 

 down onto the bottom before digging. Three spuds are provided, one on each side 

 of the bow, and one at the middle of the stern. For digging in hard-pan or similar sub- 

 stances, large teeth are bolted to the mouth of the dipper, serving to break up the 

 material. 



