436 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



give the average current speed over a more or less extended distance 

 and gives no information about the actual speed of current at any one 

 point or about its variations. 



MEASURING THE SHIP'S SPEED 



The speed of a ship is measured in a number of ways. The patent 

 log, a propeller towed behind the vessel, clear of its wake, is attached 

 by a cord to an instrument that records on dials the distance traversed. 

 Another device, the pitot log, measures the pressure differences in tubes 

 projecting from the ship's hull, and records this as speed, much as 

 the air-speed indicator of a plane. The Kenyon log measures speed 

 by the deflection of a blade projecting from the hull into the water. 

 And the speed of revolution of the ship's engines and propellers may 

 be used to judge her speed through the water when properly calibrated 

 for various conditions. Most of these methods are designed for meas- 

 uring the comparatively high speeds of ships through the water and 

 are not accurate when used to measure the slower drift of ocean cur- 

 rents past the hull of an anchored vessel. Special instruments have 

 therefore been designed for use from anchored vessels in order to 

 measure currents, both at the surface and at various depths below. 

 Lightships thus become of especial value to the oceanographer inter- 

 ested in currents at sea. 



FLOWMETERS 



Flowmeters used from stationary vessels or buoys are frequently 

 driven by means of a propeller or by a set of cups similar to those of 

 a wind gauge. These are set in motion by the water passing by. Their 

 speed of rotation is proportional to the current, and they are so ar- 

 ranged as to register the number of revolutions on a dial. The Ekman 

 typo of meter, which has been used most frequently, also has an in- 

 genious arrangement for showing the changing direction of the cur- 

 rent. The propeller is geared so as to rotate a horizontal disk con- 

 taining a single hole of the exact size to allow a small shot to pass 

 through from a shot reservoir above it. Every 33 revolutions of the 

 propeller the hole arrives in position and the shot drops through the 

 disk. Beneath the hole in the disk is a pivoted magnet carrying a 

 channel along which the shot rolls. Beneath the magnet is a box with 

 36 radially arranged compartments, each corresponding to a 10-degree 

 sector of the compass. The whole instrument is suspended so that 

 vertical fins will keep it alined with the direction of the current. Thus, 

 each time a shot drops, the compartment to which the magnet directs 

 it indicates the direction of the current. 



Current meters of the Ekman type may be suspended at intervals 

 on a long cable and in this way measurements may be made at various 



