The bathythermograph, or BT, also known as the Mechanical Bathythermograph, or MBT;[1] 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 (935 feet). Lowered by a small winch on the ship into the water, the BT records pressure and temperature changes on a coated glass slide as it is dropped nearly freely through the water.[2] 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.[1] Because the pressure is a function of depth (see Pascal's law), temperature measurements can be correlated with the depth at which they are recorded.[citation needed]