Page 413 equipment and instruments 476 



excessively. Water samples taken at such times should be preserved in glass bottles, 

 securely sealed to prevent evaporation, and stowed in a dark, cool place mitil they 

 can be tested under better conditions. The test should not be delayed longer than 

 necessary, because water samples deteriorate with age. Samples need not be retained 

 after the specific gi-avity has been measured unless they are desired by a scientific 

 organization, in which case the field party will be specially instructed by the Washing- 

 ton Office. The samples, preserved meanwhile as outlined above, should be shipped 

 at the first opportunity. 



476. Bottom Sample Devices 



A device to obtain samples of bottom materials is required so that the bottom 

 characteristics may be shown on the nautical chart. The mariner, and consequently 

 the hydrographer, is interested solely in the material on the surface of the bottom, 

 and only instruments for obtaining this information are needed in hydrographic 

 surveying. (See also 384.) 



The scientist, however, is interested in obtaining a core sample of the layers of 

 bottom sediment. Studies of these core samples disclose a wealth of important scien- 

 tific data from the 72 percent of the surface of the earth which is covered with water 

 and which, until recently, has remained hidden from the investigator. The addi- 

 tional fact that the approximate limits of the ocean basins are believed to be unchanged 

 since early geologic times also makes studies of these areas of scientific importance. 

 The sedimentary layers have been deposited in historic sequence. In the middle 

 of the ocean basins they remain undisturbed and have been deposited very slowly, 

 so that a vertical core a few feet long represents a very long period of time. The 

 geologist obtains important information from the character and thickness of the suc- 

 cessive layers. From core samples an idea may be obtained of variations in the depth 

 of the ocean in the past and also of changes in climatic conditions from a study of the 

 fossils of microscopic animals (Foraminif era) . Many minute particles of magnetic 

 materials, that have settled to the bottom, are found in bottom core samples, oriented 

 along the Une of magnetic force. A study of these minute particles, with delicate 

 electric apparatus, to determine the shift m orientation tln-ough the length of the core, 

 reveals the cyclic change of the lines of magnetic force throughout the period of time 

 represented by the deposits in th,e core. 



Such scientific investigations have no practical value in nautical chart construc- 

 tion and the hydrographer has only an academic interest in them unless he is assigned to 

 special oceanographic surveys, for which special instructions will be issued and, in that 

 case, either he or his colleagues will be thoroughly familiar with the various instru- 

 ments needed, which can be only briefly described here. 



4761. Snapper 



With the exception of the armed sounding lead (see 4664), a snapper- type device (C 

 in figs. 86 and 87) is generally used for obtaining samples of the surface layer of the 

 bottom in hydrographic surveys. This type of sampling device is especially useful for 

 obtaining samples in deep water where the bottom material will usually be washed off 

 the armed sounding lead during its ascent through the water. The snapper type of 

 bottom sampler was developed by submarine cable engineers and is sometimes called 

 a "telegraph snapper." It has two clamshell jaws that are held open by an interior 

 trigger, held in place by the pressure of a compressed helical spring around the shank of 



