MUSEUM OF COMPARATIVE ZOOLOGY. 29 
a pelagic trawl, seems to have escaped the notice of Brandt and Hensen, 
and a huge tow-net has been in use by the “ Albatross” for many years. 
I find it difficult to agree with Hensen’s statement that it ‘is non- 
sense to think one can run the tow-net at a given depth horizontally, 
and that it is therefore mere waste of money to equip for this object.” 
Certainly no one will claim to have towed along a mathematical line. 
All the variables which enter into the question as to the depth at which 
the net has been towed,—the speed of the vessel, the weight of the 
rope, its resistance as well as that of the net, and the shot which loads 
the extremity of the line, — undoubtedly make it a most intricate mathe- 
matical problem. But practically there is no such impossibility in keep- 
ing the tow-net within a very moderate distance of the required depth, 
and making constantly a careful record of the angle at which the line 
tends from the dredging boom. In our practice the net is first lowered 
vertically to the required depth, then the line is let out gradually, so 
that its length plus a small amount, taken from logarithmic tables, will 
represent the hypotenuse and catenary of the triangle along the base of 
which the net moves with a given speed which is carefully regulated by 
the angle of the rope. The shot used to sink the rope and net is a 200 
pound shot, and a 60 pound shot at the end of the net halyards. This 
is about the weight of the wire rope to a vertical depth of 300 fathoms. 
The Tanner dredging quadrant angle-indicator’ is in constant use to 
regulate the speed, and we feel satisfied from our extended practice that 
the difficulty of keeping the tow-net, say for fifteen or twenty minutes, 
at or near a given depth is comparatively slight, although the constant 
mathematical calculation of the exact position of the net is wellnigh 
impossible. The elements of error in estimating the height of the 
‘column of water passed through by the tow-net while towing vertically 
are equally great, and the same variables which Hensen enumerates 
as impossible of satisfactory reduction apply equally well to the rigid 
mathematical calculation of the height of the column through which 
the tow-net has passed. And yet he must be perfectly satisfied with 
his approximate measurement of that vertical column. 
Hensen evidently does not think it of importance to limit the tow- 
ing to a short column; his net closes after travelling 250 meters, and of 
course everything in that column is filtered through his net. Hensen 
has called attention to the danger of Chun’s net opening on its way 
down, and also closing so that it would bring up nothing, leaving it 
1 Plate XXVIII., Appendix A, Report of the Commissioner of Fish and Fish- 
ries for 1883, Washington, 1885. 
