CURRENT SURVEYS 39 
101. The general plan for observing the currents is covered by the instructions 
furnished to the chief of party. The usual procedure has been to take half-hourly 
observations, using the current pole for the surface currents and the Price current 
meter for the subsurface currents. Other types of meters are sometimes used. Before 
beginning work, the instruments should be checked to see that they are in good order. 
The distance between the graduation marks on the log line must be measured and any 
material error corrected. As this is a matter of considerable importance, a statement 
pertaining to the verification of the line should be entered in the record. The length of 
a cotton line of the type generally used varies with moisture conditions. The lines are 
wet when graduated and must be wet when the graduations are checked as well as when 
in use. 
102. The direction of the surface current is obtained in connection with observa- 
tions by the current pole. There are three different methods already described which 
may be employed—by compass direct (par. 40), by pelorus in connection with the ship’s 
compass (par. 42), or by sextant angles to a charted reference object (par. 44). In 
general, the use of sextant anglesis not adapted to continuous day and night observa- 
tions, since the reference object may frequently be obscured by fog and darkness. This 
method is, however, of value as an occasional check on the use of pelorus and compass. 
When a lighthouse is visible, sextant angles to the current pole can be observed during 
darkness by attaching a lighted flashlight to the current pole. 
103. The subsurface observations with the Price meter are usually taken at three 
depths, which are approximately 2-tenths, 5-tenths, and 8-tenths of the depth of water 
at the station. If the depth of water at the station is less than 20 feet, meter observa- 
tions at the 5-tenths depth only are usually sufficient. The meter depths are measured 
from, and should remain fixed relative to, the water surface. They are not to be varied 
as the tide rises and falls. When observing at three meter depths the meter is first 
lowered to the upper depth and held in this position for 1 minute while the revolutions 
of the meter wheel are counted and recorded. The meter is then lowered successively 
to the middle and lowest depths and similar observations taken. The observations are 
then repeated at each depth in reverse order as the meter is drawn upward. As it is a 
convenience to have the observations taken as near as possible on the exact hour or 
half-hour, it should be the aim to have the first observation at the upper depth taken 
as Many minutes before the exact hour or half-hour as the repeat observation at the 
same depth is taken after this time. The average of the two observations at each depth 
may then be attributed to the exact hour or half-hour. Pole observations if taken by 
the same observer should follow the first observation at the lowest depth before begin- 
ning the repeat observations. 
Operating Procedure Using Radio Current Meter and Buoy 
104. The general plan of the survey is outlined and the locations at which current 
observations are to be made are specified in the instructions covering the current survey. 
Before proceeding to the current stations at which buoys are to be anchored, all meters, 
transmitters, receivers and other equipment should be carefully adjusted and tested to 
insure that they are in good operating condition. KEacb buoy is anchored as near as 
practicable to the position designated in the instructions. 
105. Anchoring the buoy.—The details of anchoring the buoy and suspending the 
meter are shown in Figure 21. If the velocity of the current is expected to be less than 
