XBT
A bathythermograph, or BT, also known as a mechanical bathythermograph, or MBT, is a device that holds a temperature sensor and a transducer to detect changes in water temperature versus depth down to a depth of approximately 285 meters. Lowered by a small winch on a ship into water, a BT records pressure and temperature changes on a coated glass slide as it is dropped nearly freely through the water. While the instrument is being dropped, the wire is payed out until it reaches a predetermined depth, then a brake is applied and the BT is drawn back to the surface. Because the pressure is roughly proportional to depth according to Pascal's law, temperature measurements can be correlated to the depth at which they are recorded.