47 HYDROGRAPHIC MANUAL PaGE 400 



47. OCEANOGRAPHIC INSTRUMENTS 



The instruments chiefly used in hydrographic surveying are those designed for the 

 determination of positions and the measurement of depths. Other instruments are 

 required, however, to measure the temperature and sahnity of sea water. These essen- 

 tial data are required to compute the theoretical velocity of sound for use in R.A.R. 

 and to correct the echo soundings. Other special instruments are used to obtain 

 samples of the bottom material so that bottom characteristics may be charted. 



The thermometers used to measure the temperature of sea water are of a special 

 design that will record temperatures at various selected depths between the surface and 

 bottom. The salinity of sea water is determined from specific gravity measurements 

 made with hydrometers placed in water specimens which are trapped at various depths 

 in special instruments known as water bottles. Bottom samples are also obtained by 

 means of special instruments which enclose samples of the bottom material, which 

 may then be raised to the surface for examination and a record of the characteristics. 



Oceanographic instruments are usually made of nickel-plated brass which is little 

 afi'ected by the corrosive action of salt water in which they are frequently submerged. 

 They must be thoroughly rinsed in fresh water after use, after which they should be 

 dried with a cloth. All moving and hinged parts and screw threads should be lightly 

 oiled occasionally to ensure perfect working condition. When not in use, the instru- 

 ments should be stowed in a safe place where they will not be damaged. Thermometers 

 should be removed from the instrument frames and stowed very carefully in their 

 wooden cases. 



Oceanographic instruments should be tested before use to ensure that they will 

 function as intended. Unless an apparatus is adjusted properly it may have to be 

 raised an excessive distance before it reverses or closes as intended. It may be tested 

 by being lowered 2 or 3 fathoms below the surface to observe the action when it is 

 raised. An instrument in proper condition and adjustment should reverse or close in 

 this distance. If it does not, it should be adjusted before further use or returned to the 

 Washington Office for repairs. 



The ingenious apparatus devised by Hooke in 1660 has been utilized as the fun- 

 damental principle in all sampling devices, and few unprovements of importance had 

 been made until recent years. Recent improvements, however, have been made in 

 the design of many oceanographic instruments which not only permit quicker measure- 

 ments but also furnish more positive and accurate results. The bathythermograph 

 (473), the multiple sea sampler (4743), and the Piggot gun (4764) are examples of 

 these recent developments. Further innovations are to be expected and the hydrog- 

 rapher should be ever alert not only to improve existing oceanographic instruments 

 but also to envisage the possible development of new ones. 



471. Deep-Sea Thermometers 



A thermometer used to measure the temperature of sea water must be of special 

 design since it must not only register the temperature at any depth, but must maintain 

 the record until raised to the surface, where it can be read. There are two general types 

 of thermometers used for this purpose. The first is the reversing type. This is the 

 most accurate, and it is the easiest to use. Reversing thermometers are of two types, 

 protected and unprotected from the pressure of the water. The protected type is 

 made with or without an auxiliary thermometer to measure the temperature of the 

 enclosed column of an-. 



