RETURN 
TOSURFACE 
SEA 
SURFACE 
ECHOES 
(RAISING) 
ECHOES 
FROM 
BOTTOM 
TRANSMITTED | 
PING t 
ERAS EO TM Ee ae 
SEA 
SURFACE 
ECHOES 
(LOWERING) 
METER 
WHEEL 
SHIP'S HULL 
READINGS 
ECHOES 
STARTING 
DOWN 
— 0.25 SEC. -— TRAVEL TIME 
Figure 1A 
268 
