METHODS OF HANDLING PLANKTON NETS 199 



is thus less suitable than an instrument which provides a graph. In practice the reading 

 of the gauge was taken as the mean depth, and an estimate of the levels in which the 

 net was towing was arrived at by making an allowance (usually of about 50 m.) above 

 and below the reading. 



For all horizontal nets Stream-line Leads, as shown in Fig. 21, were used. These 

 leads are an improvement on the ordinary pattern, offering less resistance to the water 

 and giving a greater effect for the weight employed. Each lead is 56 lb. in weight, and 

 the bar, which is 3 ft. long and also stream-lined in section, is bored with a series of 

 holes so that any number of leads, up to four, can be attached to it. 



In using stream-line leads and with every type of net Swivels are necessary. For the 

 leads and light mid-water nets a simple galvanized swivel answers well enough, but with 

 trawls, dredges and large plankton nets something better is required. For such purposes 

 the ball-bearing pattern shown in Fig. 22 was used — made by Thos. and Wm. Smith, 

 Ltd. These swivels with their links are tested to 2^ tons; they are compact and pass 

 easily through the fair-leads, and in every way proved most satisfactory. 



Methods of Handling Plmikton Nets 



The engines, reels, wire ropes, nets and fittings used in plankton work on board 

 the 'Discovery' and 'William Scoresby' have now been described and figured. It 

 remains to describe the method of working. 



Vertical Nets. On ordinary routine stations one haul is made with the N 50 net 

 from 100 m. to the surface, and a series of hauls with the N 70 net, soundings per- 

 mitting, as follows: 50 m. to the surface, 100 to 50, 250 to 100, 500 to 250, 750 to 

 500 and 1000 to 750. When possible deeper hauls were taken from 1500 to 1000, 2000 

 to 1500, 2500 to 2000 or 3000 to 2500. 



In the ' Discovery ' three persons are required to work the plankton unit efficiently : 

 two scientific officers and a deck hand . Of the two officers one is on the outboard platform 

 to adjust the closing mechanism and net before descent, to despatch messengers and to 

 wash down and take the sample at the end of the haul ; the other, who is in charge of 

 operations, is inboard to control the engine, the depth reached and speed of winding, and 

 to time the despatch of the messenger. In the ' William Scoresby ' outboard platforms 

 were not necessary and only one scientific officer was required. The deck hand guides 

 the wire on the drum, pulls up the weight and holds the net for washing down. 



After an examination, when the closing rope is seen to be open to the full extent, 

 the net is lowered to the surface and the hands on the dials of the recording sheave 

 adjusted to zero. This being done, the net is lowered to the depth from which the 

 haul is to be taken. When the ship is rolling heavily — a not infrequent occurrence in 

 the vicinity of South Georgia — the wire must be let out with great care. As the ship 



S-z 



