sounding rod

As soon as they heard that heavy thump against the roof, they knew at once that this was no sounding rod, delicately probing the Sea.

Once an hour the carpenter went below with his sounding rod to sound the well: to check, in fact, how much water was getting into theCalypso as her hull worked in the big seas.

The sounding rod, a mere machine, could not bring to the surface of the earth the objects of value for the study of its internal structure, which we were about to see with our own eyes, to touch with our own hands.