T. Walker “Harpoon” water depth finder
As the brass “harpoon” headed for the bottom of the ocean, the impeller turned a screw that turned a small geared and labeled dial which also turned a larger dial on the other side measuring fathoms (1 = 6 feet) below the ocean surface. The “flap” or “stopper” here in middle movement was set first at top (or left in photo) and held there by the water descent pressure and hydrodynamics, then “fell” to the right and stopped the impeller from turning as it was brought up through the water column. I imagine Captain Nemo had a number of these “harpoons”. This one was presumably found on the tidal flats of Grand Manan Island, New Brunswick, in the Bay of Fundy, which has at last reckoning, the highest tides in the world. Usually a simple weight and knotted rope was used to determine depths, record types of bottom from retrieved beeswax on bottom of the weight, and to “mark twain” determine safe depth for passage.