54 THE WATER 



vanes A, inclined at a small angle to one another, 

 and on the other a framework carrying devices for 

 recording the velocity and direction. The velocity is 

 measured by the revolutions of the propeller B, which 

 turns on centres supported in agate cups, and is con- 

 nected by gear wheels with the pointers moving on the 

 dials on the cover of the box F. These pointers move 

 stiffly on their pivots, and can be set to zero before 

 each observation. The velocity of the current is 

 calculated from the number of revolutions in a given 

 time by a formula which must be determined experi- 

 mentally for each instrument. 



Beneath the box F is a compass, which can be 

 quickly removed and replaced, and is kept in position 

 by the thumb-screw shown below it. The needle is 

 heavy, and the arms are inclined downwards, and 

 one arm is deeply grooved on its upper surface. In 

 the box F there is a store of small shot, which are 

 released one by one by the rotation of the propeller, 

 and fall through a funnel on to the centre of the 

 needle, whence they run down the grooved arm on to 

 the compass card. The latter is divided by raised 

 partitions, and the shot falls into the compartment 

 below the end of the arm, and so records the direction 

 in which the apparatus was pointing at the moment. 

 The instrument which the writer has used has a card 

 divided at every 10 degrees, in such a way as to record 

 the direction from which the current comes, and not 

 that in which it goes. 



The meter is started and stopped by means of 

 messengers. When it is being lowered to the required 

 depth, the messenger C is not on the wire, and the 

 arm D is more nearly vertical than shown in the 

 drawing. The other end, E, is therefore slightly 

 raised, and the projection on the propeller blade butts 



