An instrument for trying the temperature of the sea at different depths. It consists of a hollow weighted cylinder in which a Six's thermometer is placed; the cylinder being provided with a valve at each end, opening upwards, so that as it sinks the valves open, allowing a free course of water through the cylinder: when it reaches the required depth the line is checked and the valves close; it is then hauled gently in, and the thermometer reaches the surface surrounded by water of the required depth, indicating its temperature.