6321 HYDHOGEAPHIC MANUAL PaGE 580 



thermometer frame is clamped high enough above it so that the thermometer holder, 

 when reversed, will not strike against the watercup. 



A sounding lead weighing about 35 pounds is generally used when serial tempera- 

 tures are observed. The water specimen cup and the thermometer frame add a con- 

 siderable weight, and the risk of overloading the wire must be avoided at all costs. 

 Depending on the strength of the sounding wire, the power of the sounding machine, 

 and economy of time in making the observations, a detachable weight with a Belknap- 

 Sigsbee specimen cylinder (see 4665 and 4744) shoidd be used in place of a lead in depths 

 greater than 700 fathoms. 



Most serial temperatures observed by the Coast and Geodetic Survey are in depths 

 less than 150 fathoms, so at least this much stranded sounding wire should be next to 

 the lead on the sounding machine used for this purpose. The wire should be renewed 

 at the first indication of appreciable wear to avoid unnecessary loss of instruments, 

 equipment, and time. 



No thermometer shall be used for serial temperatures unless it has been standardized 

 (see 4712). 



The temperature of the water near the surface may be measured with a reversing 

 thermometer in a frame attached to a hand leadline which is used to pull the thermome- 

 ter through the water until the frame reverses. 



The bottom temperature observation of a serial temperature should be made after 

 the registering sheave has been set at zero with the bottom of the lead at the surface of 

 the water. For intermediate depths the sheave should be set at zero with the reversing 

 thermometer at the surface of the water. For each observation, the reading of the 

 sheave should be checked as the instruments emerge from the water. 



The sounding-machine operator should be trained to handle the machine properly. 

 On tlie descent the machine must not be stopped suddenly and on the ascent it must not 

 be started with a jerk, because either may part the wire. The operator should allow 

 very little slack in the wire when the lead touches bottom. For temperature obser- 

 vations at intermediate depths between the surface and the bottom the operator is 

 sometimes unable to stop the machine at exactly the desired depth, with the result that 

 the depth is greater than desired. Such errors must not be rectified by reeling in a few 

 fathoms, because of the likelihood of reversing the thermometer. The sounding- 

 machine operator must be instructed to read the actual depth of each observation from 

 the sheave, irrespective of the desired depth. 



The officer-in-charge of the observations should see that the thermometer is kept 

 at each depth long enough to register the correct temperature. The time required de- 

 pends on the design of the thermometer but not more than 2 minutes is required for 

 any type of reversing thermometer now in use. 



The various depths at which observations for a serial temperature should be made 

 cannot be specified in advance for waters for which the character of the temperature 

 curve is unknown. The aim should be to obtain a sufficient number of observations to 

 define the curve accurately. In most localities more closely spaced observations are 

 required in the upper layers where the greatest temperature variation occurs, than 

 in deeper water where temperature conditions are more stable. 



For all serial temperatures, an observation shoula be made at 2 fathoms below the 

 surface, which is the approximate depth of the oscillator and hydrophone on most 

 survey ships. Where the character of the temperature curve is unknown, observations 

 should be taken at approximately 10, 20, 30, 40, 50, 75, 100, 150, 200, 300, 500 fathoms 



