Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/486

466 466 DREDGING to form but a small impediment. Fig. 2 is a section of the bore holes, and fig. 3 a plan in which the inner dotted circles repre sent the cliametejs of the broken spaces at the level of the bottom &quot;Water Level FIG. 2. of dredging. The cartiidges were formed, in the ordinary way, with canvas, and fired with Bickford s fuse. The weights of powder used for bore-holes of 4 feet, 4 feet C inches, and 5 feet were respectively 2 ft, 3 ft, and 4 Ib. The effect of the shot was generally to lift the pipes which were secured by ropes to the Y I! I/ A s y s &amp;gt;; Y V V %

&quot; i 1 H &amp;gt; ! / / &quot;%_j 10 FEET Fro. 3. rafts a few inches. Mr Edwards says that not one in a hundred shots missed fire, and these shots were generally saved by the fol lowing singular expedient : The pointed end of an iron bar, inch diameter, was made red hot, and, being put quickly through the water, and driven through the tamping as rapidly as possible, was, in nine cases out of ten, sufficiently hot to ignite the gun powder and fire the shot. The cost of each shot is calculated as follows : Use of material ..................................... 1 Labour .............................................. 03 Pitched bag for charge ........................... 00 3 Ib of powder at 5^d ............................ 1 15 ft. of patent fuse at r of a penny ........ Pitch, tallow, twine, coals, &c ................. Cost per shot 7 Each shot loosened and prepared for dredging about four cubic yards, so that the cost of blasting was Is. 9d. per yard. The cost of dredging the material, after it had been thus prepared, was 2s. 3d. , making the whole charge for removing the marl 4s. per cubic yard. &quot; One of the most recent successful combinations of blasting and dredging was that completed in 1875 by Mr John Fowler of Stockton at the river Tees, to whom the writer is indebted for the following particulars. The chief novelty was in the barge upon which the machinery was fixed. It was 58 feet by 28 feet by 4 feet, and had eight legs which were let down when the barge was in position. The legs were then fixed to the barge, so that on the tide falling it became a fixed platform from which the drilling was done. The holes were bored and charged, and when the tide rose the legs were heaved up and the barge removed, after which the shota were discharged. There were 24 boring tubes on the barge, and that was the limit which could at any time be done in one tide. The surface over which the blasting was done measured 500 yards in length by 200 yards in breadth, a small part of that surface being uncovered at low water. The depth obtained in mid- channel was 14 feet at low-water, the average depth of rock blasted being about 4 feet 6 inches. The holes, which were bored with the diamond drill, varied in depth from 7 to 9 feet, the distance between them being 10 feet. Dynamite in tin canisters fired by patent fuse was used as the ex plosive, the charges being 2 Ib and under. The rock is Oolite shale of variable hardness, and the average time occupied in drilling 5-feet holes was twelve minutes. The dredger raised the blasted rock, the cost for blast ing, lifting, and discharging at sea being about 4s. per cubic yard, including interest on dredging and other plant employed. The dredger sometimes worked a face of blasted material of from 7 to 8 feet. The quantity blasted was 110,000 cubic yards, and the contract for blasting so as to be lifted by the dredger was 3s. Id. per cubic yard. Dredging in Exposed Situations. In some cases dredg ing has to be conducted in exposed situations such as the deepening of the &quot; flats &quot; at Londonderry and the bar at Carlingford. Messrs Stevenson found that dredging at the Foyle could not be conducted when the height of the waves exceeded 2| feet ; and Mr Barton at Dundalk so far con firms this, as he estimates a swell of 2 feet as the highest to work in. Dredging on the River Clyde. An important point con nected witla this subject is the cost at which dredging may be done when conducted on a large scale. This, of course, must depend on the character of the stuff to be raised and other circumstances ; but the following information, kindly communicated by Mr James Deas, the engineer to the Trustees of the Clyde Navigation, cannot fail to be both interesting and iiseful. Mr Deas says truly that the Clyde Trustees employ pro bably the largest dredging fleet of any trust in the kingdom, in maintaining and still deepening and widening the river to meet the ever-increasing demands of the shipping trade. In the year 1871, for example, 904,104 cubic yards, or about 1,130,000 tons, were dredged from the river, of which 689,560 cubic yards were carried to sea by steam hopper barges, and 214,544 cubic yards deposited on land by means of punts. Of this 904,104 cubic yards, 345,209 cubic yards were deposit from the higher reaches of the river and its tributaries, and from the city sewers, and 558,895 cubic yards new material. The total cost for dredging and depositing was .35,448, or about 9 41 pence per cubic yard. Owing to the difference in power of the dredging machines employed, and the character of the material lifted, the cost of dredging varies much. In 1871 the most powerful machine, working 2420 hours, lifted 430,240 cubic yards of silt and so,nd at a cost of 2 60 pence per yard ; and this was deposited in Loch Long, 27 miles from Glasgow, by steam hopper barges, at 5 46 pence per yard. On the other hand, another dredger, working 2605 hours, lifted only 26,720 cubic yards of hard gravel and boulder clay, at the cost of 20 &quot;8 pence per cubic yard, which was deposited on the alveus of the river at the cost of 17 46 pence per cubic yard; another, working 18312- hours, lifted 122,664 cubic yards of silt, sand, and sewage deposit, at the cost of 5 67 pence per cubic yard, which was deposited on land at the cost of 16 40 pence per cubic yard; and another, working 2233 hours, lifted 65,160 cubic yards of till, gravel, and sand, at the cost of 5 89 pence per cubic yard, which was deposited on the alveus of the river at the cost of 9 83 pence per cubic yard. The total quantity dredged from the river during the twenty-seven years prior to 1872 amounts to 13,617,000