sounding-tube

from The Century Dictionary.

• noun A watertight tube or pipe extending vertically up from the bottom of a vessel, or from any compartment, so that the depth of water in the interior can be conveniently measured from the deck by dropping a sounding-rod attached to a line through the tube to the bottom.
• noun A glass tube, in a protecting case, lowered with sounding-lead to the bottom of the shoaler parts of the ocean, the lower end of the tube being open and the upper end closed. The air within is compressed in proportion to the depth of the ocean. The water penetrates and dissolves a red paint from the inside of the tube, so that the colored remainder indicates the amount of compression of the air and hence the depth of the water. It was invented by Sir William Thomson (Lord Kelvin) about 1860.