When the sound wave returns to the machine, you can measure
how long it took to return.
(You may notice that it's working just like RADAR, which does the
same thing with radio waves instead of sound waves.)
Even if you know how long the sound took to get to the bottom and
return to the top, you can't DO anything with this information if you
don't know the SPEED of the sound through the water. Not only
the inventory of this machine, but anyone who uses it, has to know
the speed of the sound through water in order to use the round-trip
time to calculate the depth.
