Some of the instruments used by the scientists on the Challenger expedition were remarkably simple.
Measuring the depth of water is called sounding.
Their sounding device was simply a very long rope with a heavy weight (over 100 pounds) attached to the end.
The machine lowering the rope had a counting wheel to record the length of rope as it was lowered.
The same idea was used to collect samples from the ocean floor. The weight at the end of the sounding device was a hollow cylinder.
When the cylinder hit bottom, it would fill with material from the seafloor.
When the device was pulled back to the surface, special wires attached to the cylinder would close the cylinder so that none of the sample was lost.
Their water sampler worked on the same principle.