22 U. S. COAST AND GEODETIC SURVEY 



may be considered as referring to an average depth of 7 feet. For 

 very shallow depths shorter current poles were used. 



The line attached to the pole was so graduated that the observer 

 determined the velocity in knots by the number of divisions that 

 passed over a given point during an interval of one minute. To 

 permit the pole to get a sufficient distance beyond the disturbed 

 waters near the launch and also to attain the velocity of the current 

 before observing, a stray line of approximately 100 feet in length was 

 used between the graduated line and the pole. 



The direction of the current relative to the boat was obtained by 

 means of the pelorus, a graduated dial arranged in a convenient 

 place on the boat so that the current line would pass over its center 

 as the pole was carried out by the current. The direction of the 

 vessel's head being known by the compass, the direction of the 

 current for each observation was readily calculated, corrections 

 being applied for the deviation and variation of the compass. 



Price meter. — Price current meters were generally used for the lower 

 depths. These meters indicate velocity only. Meter cups rotating 

 with the current cause an electric contact to be made at each turn of 

 the cups, or, with a device known as the pentahead used for the greater 

 velocities, at each five turns of the cups. These contacts are audible 

 to an observer with ear phones on board the boat and indicate the 

 velocity of the current by their frequency. Each meter in use during 

 the survey was tested by the United States Bureau of Standards, 

 and a rating table was prepared, showing the velocity of the current 

 corresponding to different rates of rotation of the meter cups. 



In order to expedite the raising or lowering of the meter to any 

 desired depth and at the same time to prevent it being carried side- 

 wise by any strong current, a guiding cable held vertically by a heav}- 

 weight was used. The meter, which was raised and lowered by its 

 own cable, was shackled to the guiding cable in such a manner as 

 to permit it to slide up and down as desired. 



Bifilar indicator. — To obtain the direction of the current at the 

 different depths at which the velocities were obtained by the Price 

 current meter, an apparatus known as the bifilar current indicator 

 was used. This device enabled the direction of the current at three 

 different depths to be determined simultaneously. Three vanes, each 

 supported by two parallel wires, were arranged so that they could be 

 lowered independently to the depths desired. The supporting wires 

 were connected with frames which turned with the vanes. Indi- 

 cators attached to the frames, passing over peloruses, gave the direc- 

 tion of the current relative to the vessel's heading. 



Pettersson meter. — A Pettersson current meter was also used in this 

 survey, but as only a single meter of this type was available obser- 

 vations were made principally with the Price meter and the current 

 pole. At several current stations the Pettersson meter was used in 

 connection with the Price meter with very satisfactory agreement in 

 results. 



The Pettersson meter is designed to give a photographic record of 

 both the velocity and direction of the current and will operate auto- 

 matically for a period of two weeks. A water-tight cylinder contains 

 a tiny camera with a roll of film moved by clockwork, a small elec- 

 tric lamp with batteries, a glass velocity disk with numerals inscribed 

 near the outer edge, and a glass compass disk carrying two magnetic 

 needles and inscribed with numerals to indicate direction. 



