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below the surface, and at the bottom in depths of 1,000 fathoms or less. For depths 

 greater than 1,000 fathoms, the same observations should be made down to 500 fathoms, 

 supplemented by an observation at each 1000-fathom level below the surface and an 

 observation at the bottom. (See 6342.) 



Where there are temperature gradient reversals, additional observations may be 

 required in the upper-lOO fathoms to define the temperature curve accurately, but where 

 the temperature changes are regular with reference to depth, fewer observations will 

 often suffice. After the general character of the temperature curve in an area is known, 

 depths for observations may be selected so that fewer observations are required. In 

 many areas where the deep-water temperatures are stable, observations are necessary 

 only in the layers near the surface. 



All the observations of a serial temperature should be taken at the same place, but 

 this is frequently difficult in deep water, for the ship may drift a considerable distance 

 in the 2 or 3 hours required for the observations. To approximate the same location, 

 the time required for the observations should be reduced to a minimum so the drift 

 of the ship will not be appreciable, or the vessel must be maintained at the same place 

 by maneuvering. In a serial temperature observed with only one set of instruments 

 where the depth is known by echo sounding, the temperatures in the variable upper 

 layers should be observed first before the ship drifts away from the position. Where 

 the depth is unknown, it is advisable to take the bottom observation first so those above 

 it can be spaced to the best practicable advantage; after the bottom observation, the 

 remaining observations should be from the surface downward. 



Within sight of an anchored buoy the ship may be maintained at approximately 

 the same position by bearing and distance observations on the buoy. In the open 

 ocean, beyond the visibility of any objects, approximately the same position may be 

 maintained by knowing the general direction and velocity of the current and maneuver- 

 ing the ship to maintain the same depth on the echo-sounding instrument. Maneuver- 

 in the ship during a serial temperature is generally undesirable because of the excessive 

 time required to place the ship in position for each observation. The best method is to 

 effect a reduction of the total time required for the serial. 



The use of the bathythermograph (see 473) to record the variable temperatures 

 in the upper layers of water will reduce the required time materially. For a hydro- 

 graphic party not equipped with a bathythermograph, other expedients may be used to 

 reduce the time. Two reversing thermometers may be used on two sounding machines, 

 one for the observations from the surface downward and the other for those from the 

 bottom upward. This method reduces to about one-half the time required for a serial 

 temperature, but more personnel are required and there is some risk of fouling the two 

 sounding wires. Another method is to use a number of reversing thermometers attached 

 at selected intervals on one sounding wire — a heavier sounding wire than ordmary 

 stranded or piano wire must be used to ensure against possible loss of all the instru- 

 ments through the parting of the wire. 



Surface water temperatures are generally not measured for use in echo sounding, 

 but in the determination of velocity for R.A.R. they may be required. The surface 

 temperature should be measured with a reversing thermometer as described above but, 

 if this is not practicable, a -bucket of water, uncontaminated by anj'- discharge from the 

 ship, may be drawn from the surface and its temperature measured. The latter method 

 is not entirely satisfactory, because cooling may reduce the temperature by a half 

 degree under ordinary conditions, and in winter, when the water is much warmer than 



