48 ' HYDROGRAPHIC MANUAL PaGE 418 



Pettersson plankton pump contains a small silk net through which water is forced by 

 means of a propeller type of pump. The pump is operated by gravity, a lead weight 

 unreeling a length of cable wound around the propeller shaft. The action of the pump 

 is started by a messenger, a butterfly valve closing the intake when the pump is not in 

 operation. 



For deep-sea dredging, special buckets of various sizes are made to be dragged along 

 the bottom to collect samples of the bottom material. The largest dredging buckets are 

 heavy and require special power equipment to raise them from the bottom. 



48. DRAFTING INSTRUMENTS 



Small drafting instruments, the most important of which are shown in figure 91, 

 arc an essential part of the equipment of every hydrographic party. Because of their 

 importance both to the hydrographer and to the draftsman, the necessity of taking 

 proper care of them cannot be overemphasized. To use them with maximum speed 

 and accuracy, they must be kept scrupulously clean, free from rust and corrosion, and 

 in the best possible repair. 



On board ship small instruments are particularly apt to receive harsh treatment. 

 The rolling of the ship, the salt air, and the fact that different persons must use the same 

 instruments, are contributory to the likelihood of their damage or deterioration. It is 

 not enough to require that everyone using a drafting instrument clean it and replace it in 

 its proper container. Their custody should be entrusted to a responsible officer who 

 should s"ee that instructions for their care are strictly complied with and that they are 

 kept in a locked compartment in the ship's drafting room when not in actual use. 



As protection from possible damage when the ship rolls, each instrument should 

 be kept in its separate case, if one is furnished. For small instruments such as ordinary 

 dividers, ruling pens, etc., a cigar box makes an excellent container. 



Ruling pens and other instruments in which ink is used should be wiped clean after 

 each use. All drafting instruments should be periodically cleaned to remove rust and 

 corrosion, and a light film of oil should be applied with a soft cloth. Dividers and ruling 

 pens that have become dull should be sharpened on a fine oilstone. When a draftsman 

 at a field office or on board ship needs to use the same instruments for a considerable 

 length of time, they should be issued to him and he should be held reponsible for their 

 care and accountable for damage caused by his neglect. 



481. Dividers 



Dividers are valuable accessories in measuring distances, determining speed, and 

 spacing sounding lines. In the construction and plotting of hydrographic sheets their 

 use is indispensable. 



Dividers should be adjusted so that they may be set as desired without undue 

 friction; they should not be so rigid that their manipulation is difficult, nor so loose that 

 they will not retain their setting. The points should be straight and free from burs, 

 and should be kept sharpened to a fine tapering end that will make a small round 

 puncture in the surface of paper. 



