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Below this is constructed a small reservoir (B ), and at the bottom of the tube is blown a small cavity (o), which permits of the expansion of the mercury when the thermometer is being carried with the bulb downwards.

When the thermometer is in use, it is placed with the bulb downwards, and the mercury rises and falls in precisely the same way as in an ordinary thermometer. When the moment arrives at which it is desired to take the temperature, the thermometer is reversed, the bulb then coming uppermost. The mercury column at this moment parts at (a), and falls by its own weight to the bottom of the tube indicating on the graduated scale which, reads upwards from (c) the temperature at the moment of reversal. Any mercury which is forced past the contraction (a), between the time of reversal and the moment of observation, either remains there through capillarity or falls into the reservoir (B). As an additional precaution the thermometers are now constructed with another fine contraction in the bore at (o).

To protect the thermometer against pressure, it is placed entirely in a hermetically sealed shield of glass. The shield must be strong, but need not be exhausted of air.

To counteract sluggishness some mercury is introduced in the portion of the shield surrounding the bulb, and confined there by a partition cemented in the shield around the neck of the thermometer bulb. This mercury acts as a carrier of heat from the exterior of the shield to the bulb of the thermometer.

A very simple and efficient method for reversing the thermometer, where the depths are not too great is a hollow wooden frame loaded with shot free to move from end to end, and sufficient to render the instrument just vertically buoyant in sea water. A cord is rove through the hole in the frame nearest the bulb, and the instrument is fastened by this cord to the sounding line. The thermometer will descend with the bulb downwards, but upon being pulled up, owing to the resistance through the water and consequent displacement of the centre of gravity, the instrument will turn over and come up bulb uppermost.

In sounding to great depths an accidental check upon the line may make the thermometer turn over and register.

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