The Miller-Casella thermometer
was used for most of the water temperature readings during the Challenger
expedition. Its U-shaped tube held mercury and two floating markers
that recorded the highest and lowest water temperature through which
the thermometer traveled.
Scientists spaced these thermometers at specific distances along
a line and lowered it over the side. Metal cases protected the instruments
from bumps and bangs. When the rope was hauled back in, scientists
read the thermometers and recorded the temperature for the depth
that matched the length of the rope.
Their method assumed that the coldest temperature was measured
at the greatest depth and that the warmest water was on the surface.
But soon Challenger scientists asked "What if the thermometer
had gone through a cold layer of water on its way up?" They
needed better thermometers that would reliably record temperature
at known depths.