THE COFFER DAM DREDGE.

Otago Witness, Issue 1920, 4 December 1890, Page 12

THE COFFER DAM DREDGE.

Mr W. Watsotf, engineer, has< shown,- rig drawings of a new process of lifting the^washdirt from the beds of rivers. 1 The work is effected by means of what he terms " a 'coffer dam dredge. ' A watertight J compartment, m"ade 'to any size, is sunk, through the hull of the dredge until" it rests oh the 'gravel bed in the bottom of 6he river. The gravel or wash is then raised by ireans of a steam elevator or winch, the elevator .depicted in the drawing being capable of blowing up 500 tons of water per hour. In "order to allow the coffer dam to rest evenly when placed on the sloping bank of a river, it Is provided with blinds, or sliding plateß, which can be let down so as to effectually prevent any inflow of water. Should the coffer dam rest on a large boulder,, the difficulty is surmounted by blasting the obstacle into the interior of the coffer dam, and to protect the steel plates from the shock they are lined with 3in slabs. Under ordinary oiroumstances, however, the coffer dam.'or water skin, is sunk through the gravel to a depth of from 20ft to 30ft, and the gold is cleaned up and put through the boxes. The water is next allowed to flow into the dam, the four hydraulic rams are brought Into oparation, the coffer dam is raised with a gentle lift— by means of steam within five minutes, or If the latter power is wanting, the dam can be raised up to a level with the bottom of the dredge by manual labour in the space of half-an-hour. By way of comparison, it may be mentioned, that in the year 1871, there was in use at the Dunstan a pneumatic tube, the diameter of which was sft. An area of 18 ■ superficial square feet of the river was covered, and by its means, from 4oz to 6ozof gold were raised per week. [Consequently Mr Watson contends that a coffer dam, of say 30ft x 20ft, covering an area of 600 superficial feet would in proportion to the results obtained with the pneumatic tube, raise 150oz of gold per week, with eight men at work- The cost of a cofferdam of this description is estimated not to ! exceed £2000. In the present bucket dredges, a difficulty of some magnitude is met with. The buckets sweeping along at an incline, oftentimes meet with the rough stuff of river beds, and they are unable to catch tho gold on account of it settling down beyond their reach. This contingency, the inventor of the coffer dam dredge, claims to have overcome. Of course, it yet remains to be seen whether the inven- ' tion will revolutionise the system of river dredging at present in vogue. - I