In the mid nineteenth century the British Government began to investigate the temperature of the sea on its shores, to clarify what influence had the temperature on the habits and migration of fish, and what were the seasons and temperatures more suitable for fishing.
The measurements were initially done with Six thermometers, the best available at the time, invented by Negretti & Zambra in 1857. However they did not provide satisfactory results and the same manufacturer finally did set up the model described here.
It is a mercury thermometer with cylindrical bulb and a twisted neck "A". On the shape and slimlyness of this twist depends the success of the instrument. Under "A" the tube forms a small bowl "B" and ends with a receptacle "C".
When the thermometer is held with the bulb facing down, if the temperature is high, the mercury fills the bulb, the tube and partially the receptacle "C", where remains sufficient space for mercury expansion.
In this position there cannot be any scale, because the movement of mercury is confined to the area "C". But when the thermometer is held with the bulb upward, mercury enters "A" and, as a result of its own weight, falls down into the tube, filling the receptacle "C" and the level in the pipe depends on temperature . Therefore, the scale was designed to be readable above "C".
To obtain the measurement, the thermometer is put with the bulb down and the mercury thermometer marks the temperature as normal. Wanting instead to measure the temperature at a given position - in other words, wanting to use the thermometer as a "recorder" - it is placed with the bulb upward and held in that position until the reading.
This can happen when you want, because the small amount of mercury in the bottom of the capillary is not affected by temperature changes, unless they are very pronounced. Meanwhile, the mercury will continue to shrink with the cold and expand with increasing temperature, in this second case, a small part of it will go beyond the "A" and also down to "B", but can not go further while the bulb is in high position.
The thermometer is placed in a wooden container carrying a weight, that can slide from one end to the other one. The instrument is fastened to a sounding line and the thermometer is with the bulb down in the fall, but pulling on the line, the bulb tilts and maintains that position due to the weight slipped to the opposite end, and thus indicates - or rather, "recalls" - the water temperature at the depth to which it was reversed.
The thermometer is protected against water pressure in order to be able to use it even at great depths, and is sensitive enough to obtain an accuracy up to 0.2 degrees.