16 PERCY SLADEN TRUST EXPEDITION. 
on accumulators attached to the deck at the base of the mast. This method was 
impossible on board the Sealark, but by constant care we managed to get through 
fairly successfully between Mauritius and Seychelles (Pl. 4). 
The dredges employed by us were of two shapes only, triangular and rectangular. 
The blades of the former varied from 2 to 5 feet in length, and of the latter from 
1 to G feet. The blades of the triangular dredges were set at equal angles to one another, 
some bent together round their ends and others bolted through. The former generally 
broke at the angles, and the latter straightened and drew in the same positions. We 
would ourselves recommend bolts and nuts of one pattern only, and a fair supply of 
spare blades and handles. The same applies also to the rectangular dredges. We found 
it to be almost immaterial at what angle the blade is set, so long as the dredge-rope 
can be easily let out and hauled in. Our dredges were set at different angles, and our 
experience was that, steaming at about 13 knots, the amount of warp to be let out varied 
from one and a half times the depth for a dredge with its blade at 35° to four times for 
one with a horizontal blade. We recommend a blade set at about 20°, when the warp 
required would be about twice the depth. All dredges of a pattern should have their 
blades set at the same angle so as to give the best comparative resuits. A sharp blade 
on hard ground bites into the bottom better, digs up growing organisms, and chips 
off pieces of the rock. Unfortunately it tends to bite so deeply into masses of the 
relatively soft limestone-rock around coral-reefs that it is unable to break them off, 
with the result that in spite of all manceuvring the dredge must be broken or lost. 
Probably over such ground as large a number of organisms would be secured with blunt 
blades, and much less damage would be done, Sharp blades, however, were generally 
adopted by us, because they give positive information of the nature of the bottom, which 
can be obtained by no other means. The weight of the dredge should lie in its frame; 
some of our rectangular ones were not heavy enough, so that we had to lash fire-bars on 
to them, sometimes with not very successful results, as the extra weight must be nicely 
adjusted. Our best triangular dredge, with 4-foot blades, weighed about 75 Ibs., and for 
steamship-work we would not recommend a lighter form whatever size it might be. 
Next we found by experience that at least two strong swivels should be placed between 
the warp and the dredge to prevent twisting, and that preferably they should be 
separated by a thin lead weight of about 30 lbs. Should the dredge be too light, it 
would be probably best to increase the weight in this position. “ites 
The nets generally employed were made of thick Manila cord and of 1- to 2-inch mesh ; 
canvas bags were also used for obtaining large bottom samples of mud and soft ground. 
The nets were made fairly voluminous and attached by wire to holes bored in the backs 
of the blades. Those actually employed by us varied greatly in length, but for a 
triangular dredge should not be less than four times the length of its blades, and for 
a rectangular three times; all should taper, and open at the bottom. A 20-lb. shot may 
occasionally be attached with advantage to the cord which binds up the end of the net, as 
a means of preventing the net fouling the frame when descending ; it also keeps it fairly 
taut, so that the smaller organisms cannot escape through its meshes. or rough ground 
we recommend spare nets for all dredges, and also spare pieces of netting. We ourselves 
