category. These instruments can be calibrated only by comparison 

 with a primary gauge. Some principles employed and which work 

 best for the pressure range of concern in oceanography are: elastic 

 deformation, change in the electrical resistance of conductors and 

 semi-conductors, and variation in the natural frequency of a vibrating 

 wire. 



Many other methods are used in science and industry for measur- 

 ing high pressures. The three just listed have been used with varying 

 degrees of success in oceanographic work and are considered to show 

 the most promise. The existing literature describes the successful 

 use of Bourdon tubes for depths to 2,000 feet, and the vibrating wire 

 for greater depths; however, the vibrating wire also can be used over 

 the entire range of depth. 



a. Elastic deformation 



(1) Mercury column 



The unprotected thermometer is the earliest satisfactory 

 device for measuring depth in the sea. Primitive models of this instru- 

 ment were used in the Challenger Expedition of the 1870's, and today 

 refined models remain the most accurate and reliable means for 

 measuring depth. This instrument, which is a mercury thermometer, 

 when subjected to pressure gives a fictitious "temperature" reading 

 that is dependent upon the temperature and pressure in situ . Instru- 

 ments used for this purpose are designed so that the apparent temp- 

 erature increase due to the hydrostatic pressure is about 0.01°C 

 per meter. An unprotected thermometer is always paired with a 

 protected thermometer, by means of which the temperature in situ , 

 T w , is determined. The correction to be added to the temperature 

 of the unprotected thermometer, Tu, is obtained by applying T in 

 the proper equation. The probable error of depth obtained by an 

 unprotected thermometer is about ±5 meters for depths less than 

 ljOOO meters, and at greater depths it amounts to about ±0.5 percent. 



The only satisfactory depth measuring instrument available at the 

 Hydrographic Office is the unprotected reversing thermometer. How- 

 ever, although this device can measure depth accurately, it has the 

 same disadvantages that the protected reversing thermometer has for 

 measuring temperatures (Section III). 



(2) Bellows 



The familiar bathythermograph measures and records, 



V-2 



