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 Sounding is a method of determining the depth of water by lowering a weighted line from a boat or platform to the bottom, in the case of HMS Challenger, the seafloor. The Challenger stocked 144 miles of rope for sounding and more than twelve miles of piano wire for sampling and dredging. The Challenger carried a sounding apparatus after the 1853 design of John Mercer Brooke. The twine line had a detachable weight which helped to ensure that the line struck the ocean floor and was not too derailed by the movement of the ship or the currents, increasing the accuracy of the measurements. To use this method of sounding, the crew marked distance along a rope with flags, and then, having attached the weight to the end of the rope, lowered the weight over the side. The crew then noted how fast the flags entered the water. When the weight hit bottom, the entry speed of the flags changed dramatically and the crew knew bottom had been "sounded." A line weighted with 200 kg (450 lbs) would take about 40 minutes to reach a bottom 5 km (approx 3 miles) deep. At best, Challenger depth measurements could only be expected to be accurate to within 25 fathoms (about 45m or 150 feet) because the sounding line "ruler" was only marked at each 25 fathoms. The fathom equals six feet and was a unit of depth used by the Challenger crew. The name comes from an older word for "reach." A sailor would pull in a rope as far as he could reach, about a six-foot arm-span, and know he'd measured a fathom of depth. Brooke's sounding apparatus aboard Challenger

The Baillie sounding machine was one of various sinkers used to pull the Challenger's sounding line to the ocean bottom. Like the sinker with a sampling cup, the Baille also brought back a sample from the bottom. This sinker was a hollow tube threaded through donut-shaped weights. These weights drove the hollow tube down into the seafloor and then a release catch dropped them. As the tube was pulled up, a butterfly valve swung shut and held the sample inside. The core of trapped mud or sediment was about 5 cm 92 inches) across and as much as one meter (3 feet) in length.

The Challenger crew took 360 soundings throughout the world's oceans. This tremendous effort was still only 360 pinpoints of known depth for all the 350 million square kilometers (140 million square miles) of the world's seafloor. But added to data from earlier expeditions, these soundings helped outline many major undersea mountain ranges, deep trenches and wide flat plains on the ocean floor.

This sounding line sinker had a small cup that sunk into the seafloor and brought up a sample of sediments or mud.