56 
DREDGING AT OBAN. 
ensuriiio; a unifoi’in horizontal pull upon the dredge, may he 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 the required height above the ground ;—the 
whole being made double-sided to provide for either side being upper¬ 
most when the dredge is lowered to the bottom. The present tangles 
at the bottom of the dredge-net to be retained as an additional pro¬ 
vision, and the new tangle-bar to be made as much wider than the 
present dredge-net as may be practicable for convenient handling on 
deck. 
In the “ Challenger ” Dredging it is worthy of note that the greater 
portion of the work was done with a wide trawl-net carried by a,trans- 
verse beam that was as much as 18ft. in width with 20ft. length of net; 
and this was found so superior in results to the regular dredge-net of 
4Jft. width and 4Jft. length, that the trawl-net was used for depths 
even as great as 3,000 fathoms, or miles. The two ends of the 
trawl-beam were carried about a foot clear above the surface of the 
ground by an iron-runner fixed to each end (like sledge-runners). 
The trawl-net has an objection in the rather longer time required 
for sinking it to the bottom, and it is not so suitable for rocky or 
uneven ground as the narrow dredge ; but it was used almost con¬ 
stantly during the latter part of the voyage of the “ Challenger.” In 
the deep-sea dredging (generally about three miles depth), the time 
required for lowering and hauling up the dredge was so great that only 
a single dredging operation could be effected in each day; the lowering 
of the dredge took about 3 hours, it was then allowed to remain at 
the bottom from to 2 hours, and 4 to 5 hours was taken for hauling 
up, the average I’ate of hauling up being 1ft. per second, or 10 fathoms 
per minute. The total number of dredgings effected in the 3^ years 
that the “ Challenger ” voyage lasted was consequently not more than 
304, these being nearly all deep-sea dredgings, extending to an extreme 
depth of 3,950 fathoms, or 4^ miles. 
In future dredging excursions of the Society it appears also desir¬ 
able to consider the employment of a small-sized dredge in addition to 
the ordinary dredge, and so smali in size that it can be hauled in 
readily by a single hand, and can be quickly and conveniently used as 
a trial dredge without involving the delay and labour of lowering and 
raising the large dredge. Such a dredge, of only seven inches width, 
and requiring a rope only as large as a common lead pencil, has been 
very successfully used down to a depth as great as 60 fathoms by Mr. 
David Robertson, of Glasgow, in his important dredgings for Foramin- 
ifera on the coasts of Scotland and Norway. 
In reference to the preserving of specimens, it is very desirable that 
on future occasions (as pointed out in the Report on the Pennatulida 
from Oban) some few specimens of each object—fragments will suffice 
