48 OBSERVATIONS IN PHYSICAL OCEANOGRAPHY 



like a wide, inverted V. The upper side of one arm of the bar is trough- 

 shaped, so that a ball dropping through a hole in the center of the Ud 

 of the box runs down this trough and falls to the bottom of the box, which 

 is divided into thirty-six compartments, each corresponding to an angle 

 of 10 degrees and marked N, N 10° E, N 20° E, and so on. The compass 

 box is rigidly connected to the vane of the meter, but the magnets of the 

 compass adjust themselves in the magnetic meridian. The compartment 

 into which the ball drops therefore indicates the direction of the vane — 

 that is, the direction of the current at the moment the ball fell. In 

 general, several balls fall during one observation, since one ball drops for 

 each thirty-three revolutions or less of the propeller. The average 

 direction of the current is obtained by computing the weighted mean 

 according to the distribution of the balls. If the direction has varied 

 widely during the period of observation, the average direction will be 

 uncertain or even indeterminate, in which case the average velocity as 

 computed from the revolution of the propeller has no meaning. 



