On this day in history,...

...in 1938, the Journal of Marine Research published a paper by Athelstan F. Spilhaus describing a new instrument he had designed: the bathythermograph (BT). This device marked a significant technological breakthrough in oceanography, enabling the continuous recording of temperature with depth during a single cast—something not possible with the discrete measurements of Nansen bottles and reversing thermometers.

Spilhaus’s invention consisted of a compact mechanical apparatus that could be towed from a ship. It included a temperature-sensitive bimetallic strip and a pressure-sensitive element, both linked to a stylus that traced a curve on a small smoked glass slide. This allowed oceanographers to visualize the temperature profile of the upper ocean in near real time, without the need to retrieve multiple water samples or rely on indirect calculations.

The bathythermograph proved especially valuable during World War II, when it was adopted by the U.S. Navy to detect thermoclines—sharp changes in temperature that submarines used to evade sonar detection. It quickly became a standard instrument for both military and civilian oceanographic operations.

Later developments led to the creation of the Expendable Bathythermograph (XBT) in the 1960s, which no longer required retrieval and transmitted temperature-depth data electronically via a thin wire as it descended through the water column. Although today CTDs and autonomous platforms have taken the lead in ocean profiling, XBTs are still used in certain monitoring programs due to their simplicity and cost-effectiveness.

The publication of Spilhaus’s 1938 paper marked the birth of a new era in ocean observation. The bathythermograph fundamentally changed how scientists measured and understood the thermal structure of the ocean, paving the way for more advanced instrumentation and modern physical oceanography.

Sources

•    Spilhaus, A. F. (1938). A Bathythermograph. Journal of Marine Research, 1(2), 123–131.
•    Montgomery, R. B. (1954). Bathythermograph Measurements. Deep-Sea Research, 1(1), 1–14.
•    NOAA History: https://www.history.noaa.gov/
•    National Research Council (1990). The Ocean's Role in Global Change: Progress of Major Research Programs. National Academies Press.