On Sixe&rsquos Principle

Thermometers for ascertaining the temperature of the sea at various depths are constructed to register either the maximum or minimum temperature, or both. The principle of each instrument is that of Sixe. There are very few parts of the ocean in which the temperature below is greater than at the surface, except in the Polar Seas, where it is generally found to be a few degrees warmer at considerable depths than at the surface. When the instrument is required to register only one temperatu e, it can be made narrower and more compact—a great advantage in sounding; and with less length of bulb and glass tube, so that the liability of error is diminished. Hence, the minimum is the most generally useful for deep-sea soundings. These thermometers must be sufficiently strong to withstand the pressure of the ocean at two or three miles of depth, where there may be a force exerted to compress them exceeding three or four hundred atmospheres (of 15 lbs. to the square inch).

Many have been the contrivances for obtaining correct deep-sea indications. Thermometers and machines of various sorts have been suggested, adopted, and eventually abandoned as only approximate instruments. The principal reason for such instruments failing to give correct or reliable indications, has been that the weight or pressure on the bulbs at great depths has interfered with the correct reading of the instruments. Thermometers have been enclosed in strong water-tight cases to resist the pressure; but this contrivance has only had the tendency to retard the action, so much so as to throw a doubt on the indications obtained by the instrument so constructed.

The thermometers constructed by Messrs. Negretti and Zambra for this purpose do not differ materially from those usually made under the denomination of Sixe’s thermometers, except in the following most important particular:—The usual Sixe’s thermometers have a central reservoir or cylinder containing alcohol; this reservoir, which is the only portion of the instrument likely to be affected by pressure, has been, in Negretti and Zambra’s new instrument, superseded by a strong outer cylinder of glass, containing mercury and rarefied air; by this means the portion of the instrument susceptible of compression, has been so strengthened that no amount of pressure can possibly make the instrument vary. This instrument has been tested in every possible manner, and the results have been highly satisfactory, so much so as to place their reliability beyond any possible doubt.

The scales are made of porcelain, and are firmly secured to a back of oak, which holds in a recess the bulb with its protecting shield, and is rounded off so as to fit easily and firmly in a stout cylindrical copper case, in which the thermometer is sent down when sounding (see fig. 69). The lid of the case is made to fit down closely, and water-tight. At the bottom of the case is a valve opening upward; and the lid has a similar valve. These allow the water to pass through the case as the instrument sinks, so that the least amount of obstruction is offered to the descent. At the lower end of the case is a stout brass spring, to protect the instrument from a sudden jar if it should touch the bottom while descending rapidly. As the instrument is drawn up, the valves close with the weight of water upon them, and it arrives at the surface filled with water brought up from its lowest position. The deep-sea thermometers used in the Royal Navy are of this pattern.