CHAP. XIX.] UNDER-CURRENTS 413 



is also an unknown quantity, but will always have the effect 

 of retarding the motion of the submerged drag. The rate 

 therefore recorded, by the movement of the surface buoy, will 

 always be less than the true rate of the under-current. 



We do not imagine that the apparatus described above may 

 not be much improved upon, but we give it as a starting-point 

 for any officer employed in future investigations of a similar 

 character. 



Several instrument makers now turn out " Current Meters " Current 

 of various forms. Doubtless these could be, with a little ® "' 

 ingenuity, adapted to sea-work, at least to show the true rate 

 of an under-current. 



A deep-sea current meter, devised (1876) by Lieutenant ^i^^^''^ 

 Pillsbury, U.S.N., has, with several modifications, been used Meter, 

 with success on many occasions, notably by the U.S. Coast 

 and Geodetic Survey steamer Blake in the investigation of the 

 Gulf Stream. The instrument is first lowered to the required 

 depth, and when ready is put into action by means of a heavy 

 weight, or messenger, travelling down the supporting line and 

 striking on a metal plate, thus closing the jaws of the levers 

 and enabling the instrument to begin working. The rudder 

 is then free to revolve inside the framework and take up the 

 direction of the current ; the small cones can revolve on their 

 axis and register the number of revolutions, while the compass 

 needle is released and free to take up the north and south line. 

 On the despatch of a second messenger, which strikes on the 

 top of the first and forces the jaws of the levers open, every part 

 of the machine is simultaneously locked. Having noted the 

 exact time of starting each of the messengers, the time during 

 which the instrument has been working at the required depth 

 is known, and from tliis the velocity of the current can be 

 calculated, the number of revolutions having been recorded, 

 while the direction is shown by the angle between the compass 

 needle and the direction of the rudder. 



The instrument is shown in Fig. 68. A A are the jaws of the 

 levers through which the first messenger passes and strikes 

 on the metal plate B. The force of the blow is sufficient to 

 press B down, thus bringing the jaws as close together as 

 possible, and putting the meter into action. The second mes- 

 senger, falling on the fii'st, opens the levers again, and prevents 



