Bathythermograph MkV by S.G. Brown Limited, Watford, England, c. 1958

The bathythermograph is an instrument widely used to record the ocean temperature at various depths. It would be lowered from a moving vessel on a specially constructed cable and recorded the ocean temperature and the associated water pressure. Until the development of this form of thermometer in the 1930s, the thermocline (the layer of rapid temperature change between surface and deep water) had not been understood. The bathythermograph was able to show the structure of this layer more readily than could be achieved by measurements using a water bottle and thermometer. This instrument was made by S G Brown.