at its upper end should be passed outside the 

 guard rod, through 1 slot in the base of the 

 frame, and then slipped over the longer of the 

 hooks when rod (A) is rotated for the first 

 time. The wire from the other messenger 

 should be about 13 inches long. It is passed 

 through the other slot and hung on the short 

 hook when rod (A) is rotated to its final 

 cocked position. To make a quantitative 

 plankton investigation, it is necessary to record 

 the reading of the cyclometer before the sampler 

 is lowered into the water. It is read again when 

 the sampler is brought aboard after an observa- 

 tion. The difiFerence between the 2 readings 

 multiplied by 4 is the approximate number of 

 liters of water filtered by the sampler. 



The Clarke-Bumpus plankton sampler is 

 attached to the wire by a hinged clamp, a spring 

 pin, and a gate lock. To attach the sampler to 

 the wire open the hinged clamp (F), slip it 

 around the supporting wire, then close and 

 tighten it by means of a wing nut so that the 

 clamp cannot slide on the cable. Next, snap 

 the top of the frame of the sampler onto the 

 cable by means of a spring pin (G) and close 

 the gate lock (H) around the neck of the 

 clamp. Make sure that the hinged clamp (F) 

 is closed sufficiently to allow the gate lock (H) 

 to swivel freely around it. If messengers are 

 to be placed below the sampler, snap them 

 around the wire beneath the clamp, being sure 

 that messenger hoop wires pass outside the 

 guard rod and that the cutout surface of each 

 messenger is uppermost. When towing the 

 sampler, a slight bend is produced in the wire 

 where it passes over the spring pin (G) ; the 

 magnitude of the bend depends upon the 

 amount of horizontal resistance and the size of 

 the weight at the end of the cable. If messen- 

 gers fail to strike the trigger (E) when the 

 heaviest weight is used, the trigger should be 

 extended by sliding out the adjustable tip 

 of the trigger. 



8-14 How To Secure the Net After A 

 Tow. — After each use of the sampler, it must 

 be rinsed completely with fresh water. The 

 salt water should be removed from the metal 

 parts, and any plankton remaining on the net- 

 ting must be washed off. The sampler should 

 be hung in a protected place so that the netting 

 or meter will not be damaged. 



8-15 Preserving and Storing Planltton Sam- 

 pies. — The instructions are given in sections 

 8-32 and 8-33 for preserving and storing plank- 

 ton samples. 



8-16 Maintenance of tlie Clarke-Bumpus 

 Sampler. — One of the primary precautions to 

 be taken for keeping the sampler in good opera- 



ting condition is to see that all movable parts 

 are well lubricated. For a sampler which will 

 be used in salt water, skunk oil is the best 

 lubricant for all exposed parts. After each 

 use, the sea water must be washed off the 

 sampler, especially from the cyclometer. 

 Then dry and lubricate the sampler, clamp, and 

 messengers with a suitable light machine oil 

 or tectyl. The cyclometer may be kept full of 

 oil if so desired. 



If a sampler is to be used in salt water, it is 

 advisable to have all parts of the instrument 

 nickel-plated at the time of construction to pre- 

 vent corrosion. 



8-17 Spare Parts Needed. — The following 

 are the spare parts which may be required when 

 making observations with this sampler: 



a. Silk nets of various mesh size. 



b. Plankton buckets of the same size mesh 

 as the netting. 



c. Special Clarke-Bumpus messengers, 1 

 spare. 



d. Supporting clamp, 1 spare. 



e. Preserving jars for storing and shipping 

 plankton samples collected. Pint or half-pint 

 mason jars are recommended. 



f. Formalin (37-40 percent solution of form- 

 aldehyde) . 



g. Sodium bicarbonate — one box. 



h. Linen for patching — 1 square yard. 



i. Silk thread for patching — one spool. 



8-18 THE MIDWATER TRAWL.— The 

 Isaacs-Kidd midwater trawl was developed at 

 the University of California, Scripps Institution 

 of Oceanography. It is capable of collecting 

 some of the larger and more active animal forms 

 found in the ocean. As implied by its name, 

 the trawl was primarily designed for use in 

 midwater, that is, ocean water below the surface 

 layers. An ordinary net will surface behind the 

 towing vessel unless hauled at extremely slow 

 speeds. To counteract this tendency, the mid- 

 water trawl has an inclined plane surface rigged 

 in front of the net entrance to act as a depressor, 

 in a manner opposite to the elevating action of 

 a kite surface. 



The midwater trawl has been manufactured 

 with both a 10- and 15-foot mouth, and in 31- 

 and 72-foot lengths, respectively. The follow- 

 ing descriptions and instructions will apply pri- 

 marily to the 10-foot model, but with minor 

 modifications they will also fit the 15-foot model. 



8-19 The Net Assembly. — The net is essen- 

 tially an asymetrical cone of 2}^-inch stretch 

 mesh with a pentagonal mouth opening and a 

 round cod end. From a point 3 feet in front of 

 the cod end to the terminus of the net, an ad- 

 ditional netting of K-inch stretch is attached as 



82 



H. O. 607 



