4762 HYDROGRAPHIC MANUAL Page 414 



the instrument. The trigger is formed of two metal pieces hinged on the inside of each 

 jaw, the end of one containing a fded groove and the other a chisel edge. They fit 

 together when the jaws are open and are arranged so that their point of contact is slightly 

 above the line between the two hinges. 



The correct adjustment of the tension of the spring is important and must be reached 

 by trial. When the tension is correct the trigger will always be released when the jaws 

 strike the bottom but they will be held firmly closed together at other times. The tension 

 may be increased or decreased by moving a nut, which compresses the spring, along a 

 screw thread on the shank. 



The shank of the sampler extends above the upper end of the spring so that it can 

 be inserted in a sounding lead. A lead weighing 35 pounds or more should be used for 

 deep soundings (see 466). 



This type of bottom sampler occasionally fails to return a sajnple from sand or rock 

 bottom. If a pebble, shell, or other hard object lodges between the jaws to prevent 

 them from closing firmly, the bottom sample will frequently be washed out during 

 the ascent to the surface. A hard bottom will generally dent the jaws badly so that 

 they must be repaired before subsequent use. The snapper is very effective for return- 

 ing samples of mud or soft bottom. 



Snappers are obtainable in two sizes. The small si^e has a capacity of about 15 cc, 

 the larger having approximately six times that capacity. The small snapper is regularly 

 carried in stock at the Washington Office and will be found satisfactory for all hydro- 

 graphic surveys. The large size must be ordered specially from the manufacturer but 

 should not be needed unless large samples of bottom material are required for 

 preservation. 



The snapper type of sampler is also adapted for use with a detaching weight of 

 special design. This weight is cast in pear-shaped halves, which are held in a seizing- 

 wire sling attached to a Sigsbee releasing device, similar to that embodied in the Bel- 

 knap-Sigsbee specimen cylinder (see 4744). When the snapper strikes the bottom the 

 weight is detached and falls apart, falling clear of the snapper, and is left on the 



bottom. 



4762. Coring Instruments 



Coring instruments are designed to obtain samples of sedimentary layers from the 

 bottom of the ocean. To be of value the instrument must return an undisturbed sample 

 with the layers in their true relation to each other from top to bottom. 



The usual device for obtaining bottom cores is simply a weighted metal tube with 

 a cutting edge which is driven into the bottom by impact on striking. Detachable 

 weights are used with some of these instruments, but with others fixed weights are used. 

 The latter are difficult, inmost cases, to withdraw from the bottom and raise from great 

 depths. Most coring instruments require more powerful sounding machines and stronger 

 wire than are customarily used in hydrographic surveying. The impact type of coring 

 instrument weighs 400 to 500 pounds. The length of the core sample obtained depends 

 on the character of the botton, but cores as long as 18 feet, the maximum length of the 

 coring tube, have been obtained from a soft mud bottom. 



4763. Vacuum Lead 



A recently developed (1941) coring instrument, known as the "vacuum lead," makes 

 use of hydrostatic pressure to force a tube into the bottom sediment. This instrument 

 consists of a coring tube, 18 feet long, which fits into a hollow cast-iron sphere to which 



