Page 403 equipment and instruments 4714 



temperature and the thickness and size of the protecting glass case. The process may be hastened by 

 submerging the reversing thermometer in a bucket of water and allowing the temperature of the column 

 of air to reach the temperature of the water, which will require about 4 or 5 minutes. The temperature 

 of the enclosed column of air is easily determined when a special reversing thermometer,, that has an 

 auxiliary thermometer inside the protecting case, is used. With this type it is only necessary to take 

 a simultaneous reading of the reversing thermometer and the auxiliary thermometer. Regardless of 

 the method used to obtain the temperature of the enclosed column of air, the graph (fig. 85), is entered 

 with this temperature on the abscissa scale and where it intersects the temperature of the detached 

 thread (observed temperature) the correction is read from the ordinate scale. 



4714. Accuracy of Observations 



Precisely measured temperatures are not necessary for hydrographic surveys. An 

 accuracj'' to a tenth of a degree centigrade is satisfactory for all purposes. This is 

 equivalent to an accuracy of one-half meter per second in the velocity of sound, which is 

 adequate for the computation of the horizontal velocity and for correcting echo sound- 

 ings. In water of moderate depth (100 fathoms or less) this accuracy may be obtained 

 if the reversing thermometer is raised rapidly and the temperature read immediately 

 after the thermometer reaches the surface, before sufficient time has elapsed for the 

 temperature of the enclosed column of air to chacge materially. The observed reading 

 should, however, be corrected for the intrinsic error of the thermometer, which will be 

 obtained from the corrections based on actual tests (not the computed ones) made by 

 the National Bureau of Standards. If the temperature is not read immediately, or if 

 the observations are made in deep water, the temperature of the enclosed column of air 

 should be determined and the total correction made. 



Accurate water temperatures are sometimes required for scientific studies but, 

 when required, the Chief of Party wUl be so notified in the project instructions. For 

 such use additional precautions are necessary to ensure that the temperatures are accu- 

 rate to a few hundredths of a degree centigrade. A special reversing thermometer, 

 graduated to tenths of a degree and containing an auxiliary thermometer, should be 

 used when accurate temperatures are specified. 



4715. Unprotected Reversing Thermometer 



There are also deep-sea reversing thermometers of an unprotected type, which are 

 not sealed in glass cases to protect them from the pressure of the water. These are not 

 used by the Coast and Geodetic Survey, but are sometimes used in oceanographic 

 studies. An unprotected reversing thermometer will give a fictitious temperature 

 higher than the true temperature because of the compression of the glass. Used in 

 combination with a protected reversing thermometer at the same depth, the difference 

 in temperature of the two is a measure of the pressure, and from these data the depth 

 at which the thermometers were reversed may be computed. This method is used 

 to determine the depths at which observations are made, especially during rough 

 weather when the sounding wire does not remain vertical. 



4716. Insulated Water Bottle 



Water temperatures may also be measured with an ordinary type of accurately 

 graduated thermometer enclosed in an insulated container such as the Nansen-Petters- 

 son water bottle. The container is made of metal with interior insulation. It has an 

 upper and a lower valve which remain open during the descent of the bottle, but which 

 are closed at the desired depth by means of springs, released by a messenger. The ther- 



