366 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
brass rod is secured iuside of the scale to strengthen the frame and prevent its warp- 
ing, also to protect the i)endnlum point which swings between it and the scale. 
To use the dredging quadrant grasp it in both hands, straight edge up, knob 
to the right; see that the pendulum swings freely; then take a favorable position 
(cut 68), cast the eye over the straight edge of the quadrant, inclining it to tlje angle 
of the rope, at the same time sweeping it back and forth until they are parallel ; then 
lock the pendulum by i>ressing the spring catch E with the thumb of the right hand. 
The angle of the rope from the perpendicular may then be read from the scale. 
The following illustrative example explains the principle of the dredging quadrant 
and its inactical application : 
Given the depth, 1,000 fathoms, and the angle 10°, what is the scope of dredge 
rope required to insure che landing of the trawl on the bottom'? 
Enter Table II, Bowditch, or any table for the solution of j)lane right triangles, 
with 40° as a course, and tind the depth, 1,000 fathoms, in the difference of lati- 
tude column (taking one-tenth of the amount), 100.4 being the nearest number. 
Opiiosite to this, in the distance column, is 131, which being multiplied by 10 gives 
1,310 fathoms, the hypotenuse of the right triangle we have constructed. As the rope 
has a catenary curve it is necessary to make an allowance in order to insure the trawl 
reaching and remaining on bottom. Experience teaches that about 200 fathoms is 
sufficient Avith above depth and angle; therefore, with a scope of 1,500 fathoms, and 
the angle of the rope maintained between the limits of 35° and 40°, a successful haul 
may be anticipated so far as the landing of the trawl on the bottom is concerned. 
Tlie speed at which it can be dragged varies from 2 to 2^- knots per hour, depend- 
ing upon the state of the sea, the currents, and the character of bottom. It can be 
regulated after a little iiractice so as to conliue the angle of dredge rope Avithin the 
limit of 5°; hence there is a wide margin as to its scojie, Avhich is governed largely by 
the speed at which it is desired to drag the trawl or dredge over the bottom. 
SURFACE AND INTERMEDIATE COLLECTING. 
The surface tow net was among the first devices of the naturalist for collecting 
minute animal and vegetable forms on the surface, and the same apparatus has been 
used at intermediate depths, although its range was confined Avithiu uarroAv limits, 
usually but a few fathoms, and even then it was not entirely satisfactory, as specimens 
would naturally find their Avay into the net Avhile it Avas being hauled to the surface, 
the exact depth of their habitat remaining a mystery. 
The ring of the surface toAv net in common use is from 12 to 18 inches in diameter, 
made of )^-inch iron or brass rod. The best nets are of silk gauze, or bolting-cloth, 
although they may be made of cheese-cloth or other suitable material. They are 
usually toAved with a small line either astern or over the side Avhen the vessel is 
moving slowly through the Avater. 
The dip net has been in constant use on board the Albatross. Its ring resembles 
that of the surface toAv net, but is usually made of heavier Avire, and it has a shank 
Avhich is inserted into a staff, preferably a bamboo pole of sufficient length. The net 
is of silk liolting-cloth. 
This device may be used at any time Avhen the vessel is lying without headway 
or moving very slowly through the water. Its greatest achievements have been in 
connection with the electric light. At night, preferably from one to three hours after 
dark, the A'^essel lying broadside to the Aviud and Avithout headway, an ordinary 
