84 DREDGING AT OBAN. 



satisfactory is, that the dredge came up on several occasions only very 

 imperfectly filled, and appeared to have not been dragged properly 

 along the sea bottom, and to have been lifted off the bottom by the 

 oblique pull of the rope. In the " Challenger " deep-sea dredging, 

 where the difficulty from this cause was enormously increased by the 

 dredging being carried to a depth of from 2^ to 4J miles, a special con- 

 trivance was used for overcoming this difficulty, and a weight of 1 J cwt. 

 was attached to the rope at 200 to 500 fathoms' distance in advance of the 

 dredge (or a distance equal to about l-8th of the depth being dredged). 

 This "messenger" weight trailed along the sea bottom, and caused 

 the pull upon the dredge to be under all circumstances in a directitin 

 parallel to the bottom and close down upon the ground, tlius keeping 

 the dredge at all times in the best position for its work. As a con- 

 sequence of this arrangement, the risk was somewhat increased of the 

 dredge getting jammed by some obstruction at the bottom, and when- 

 ever that occurred it became necessary to ease the rope by letting it 

 slip for a certain distance, until the steamer could be backed over the 

 dredge in order to clear it from the obstruction. The circumstance of 

 the dredge becoming jammed was instantly ascertained by means of a 

 spring-tension apparatus in the hauling tackle, the guiding pulley over 

 which the rope was led to the dredge being suspended by a set of 80 

 strong india-rubber bands (similar to door springs), which were 

 capable of stretching as much as lift, before reaching the strain at 

 which the rope would break (2^ tons) ; and due warning was thus given 

 by a sudden increased stretch taking place whenever the dredge got 

 fast. In the Oban dredgmg this object was aimed at by feelmg the ten- 

 sion of the rope by hand from time to time, to detect any undue 

 resistance, and also to ascertain whether the resistance was sufficient 

 for indicating that the dredge was dragging properly on the ground. 



In order to prevent the injury of the specimens by the scraping lip of 

 the net striking them before they can be caught by the tangles, it appears 

 requisite for the tangles to be fixed separately farther up the rope in ad- 

 vance of the net, so that when the dredge is dragged along, the bottom 

 shall be first liipitlt) nwept by the tangles before being scraped by the 

 dredge ; and it is also requisite that the cross-bar carrying the tangles 

 shall not itself drag upon the ground so as to cause similar mischief to 

 that previously done by the dredge scraper, but shall be propped up 

 sufficiently high above the ground to prevent this, but not so high as 

 to risk the tangles floating clear off the ground. 



It is now suggested that the above objects, and also the other re- 

 quirement of a "messenger" weight in advance of the dredge for 

 ensuring a uniform horizontal pull upon the dredge, may be both con- 

 veniently effected by having a cross-bar carrying the tangles attached 

 to the rope in advance of the dredge (say three fathoms in advance for 

 twenty-four fathoms depth of dredging), and having a weight fixed to 

 each end of this bar, of such a form as to drag upon the ground whilst 

 holding up the bar at tlie required height above the ground ; — the 

 whole being made double-sided to provide for either side being upper- 



