Perhaps some of our readers may have seen a description of a form of thermometer devised by MM. Negretti and Zambra for the purpose of ascertaining the temperature of the ocean at great depths. This consisted of a tube bent into the shape of a siphon, which when it had reached the desired depth was made, by means of an ingenious arrangement, to pour all the mercury found above a certain point near the reservoir into the second arm of the siphon. This second arm, which, like the other, was a capillary tube, carried a scale of divisions on which might be read the temperature of the depths to which the instrument had been lowered. This thermometer gave all the results that might have been expected. The ship Challenger during its polar expedition had on board a certain number of these instruments. The report of Capt. G. S. Nares made to the English Admiralty describes all the benefits that we may hope to reap from a serious study of the temperature of the ocean at different depths, and not the least of these are those that pertain to the fishery interest. Notwithstanding the good results given by this instrument, its inventors have endeavored to render it still more practical and more within the reach of all by diminishing the cost of construction, and increasing its compactness.

Fig. 1 represents the thermometer isolated from its case. It is an ordinary thermometer furnished at A with a little device that M. Negretti has already made use of in the construction of his larger instrument, and which allows the liquid to run from the reservoir into the capillary tube when the temperature rises, without letting it flow back when it lowers, if moreover the precaution has been taken to incline the tube slightly, reservoir upward. At B there is a bulge in the tube in which a certain quantity of mercury may lodge; this bulge is placed in such a way that the mercury resulting from the dilatation of the reservoir may come to it and continue its ascension in the capillary tube when the reservoir is down (the thermometer being vertical), but cannot get out when the reservoir is upward.

We should add that these thermometers are constructed so as to give the variations of temperature within determined limits.

The small reservoir, B, is indispensable to the well working of the apparatus; for in seeking the temperature at a certain depth the instrument may, on being drawn up, pass through warmer strata, and it is necessary, therefore, to provide the reservoir with a means of diffusing the small quantity of mercury resulting from this excess of temperature. The tube has also a small bulge at its upper extremity at C.

The thermometer is placed in a small wooden case having a double bottom throughout its length. In this double bottom are placed a certain number of lead balls that can run from one end of the case to the other, and of sufficient weight to render the instrument buoyant in sea water. To use the apparatus, one end of a cord is passed through a hole in the case under the reservoir of the thermometer, and the other end is tied to the sounding line at a certain distance from the lead (Fig. 2). While the line is descending the thermometer will remain reservoir downward (Fig. 2); but when it is again drawn up the thermometer case will take the position indicated in Fig. 3, and the column of mercury breaking at A will fall into the capillary tube, the divisions of which, as will be seen at Fig. 1, are reversed.

As to the thermometer itself, it is important to protect it against the pressure which becomes so considerable at great depths; to do this the reservoir is surrounded by an envelope of thick glass about three quarters full of mercury. The mercury serves to transmit the temperature to the reservoir, and should the exterior envelope yield to the effects of pressure, the reservoir proper would not be affected, the mercury not exactly filling the annular part which surrounds it.