Methods of Observation 13 



Reversing thermometers. — The reversing thermometer is mounted on the 

 samphng bottle. The thermometer is so contrived that when it is turned 

 upside down the mercury column breaks. When the thermometer is brought 

 to the surface the reading must be corrected, because the temperature at 

 which the mercury is broken is usually different from that at which it is 

 read on deck. 



These thermometers are of two distinct types, which are used in pairs; 

 that is, two thermometers, one of each type, are mounted on the same 

 bottle. One type is protected from the hydrostatic pressure by an exterior 

 glass shell (thermal contact with the outside is maintained by a pool of 

 mercury); the other type is unprotected from the pressure, and hence 

 registers depth because the mercury bulb is squeezed. The deviation 

 amounts to about 0°01 C. per meter. The standard of accuracy of each type 

 is + 0°01 C. Reversing thermometers and reversing bottles are designed for 

 use with the ship hove to or steaming very slowly against the current or 

 into the wind. They are necessary for making geostrophic current calcu- 

 lations. 



Bathythermograph. — The bathythermograph is an instrument designed 

 for use from a ship under way. A smoked-glass shde is mounted on a 

 spring-loaded bellows so that it moves under variations in hydrostatic 

 pressure. A bourdon thermal element moves a needle over the slide at 

 a right angle to the direction of the pressure-induced motion. The scratch 

 on the smoked slide is therefore a plot of temperature versus depth. The 

 standard of accuracy in a model which works to a depth of 900 ft. is 10 ft., 

 and + 0°2 C. Models for other depth ranges are also available. 



Bathypitotmeter. — A promising device for towing at low speeds, to make 

 simultaneous recordings of the velocity of water relative to the instrument, 

 the temperature, and depth, was devised by Malkus (1953). The record is 

 made on a moving waxed-paper strip. This instrument is capable of making 

 a determination of a complete vertical sounding in about two hours. The long 

 length of cable, which is payed out when the ship is under way, apparently 

 isolates the instrument from the irregular pitch and roll motions of the 

 ship, which normally cause spurious readings on velocity meters suspended 

 vertically from a drifting ship. 



Propeller-type current meters. — There are various kinds of current meters 

 of the propeller type, from the early Ekman meter to the recent Watson 

 meter. They record velocity and direction, but they take so long a time to 

 operate that they are not often used for Gulf Stream studies. 



All velocity meters measure water velocity relative to the ship, but the 

 ship's velocity relative to the earth is seldom known adequately from 

 navigation, even in areas with good Loran coverage (where radio fixes good 

 to 1/2 mile are available at any time). Therefore there is always an 



