The apparatus for detecting differences in salinity by measuring the conductivity of the water is called a “salinometer,” and its most perfect form, known as the heat-compensated conductivity salinometer, is due to Dr. Myer Coplans. Fig. 43 shows a diagram of this interesting piece of apparatus, which is most ingeniously devised. Two insulated electrodes of copper, with platinum points, are suspended in a U-tube through which the sea water passes continuously, as indicated in the diagram. A steady current is passed through the column of water between the two platinum points, and the conductivity of this column is measured continuously by very accurate instruments. Variations in the conductivity, indicating corresponding variations in the saltness of the water, are thus shown immediately; but before these indications can be relied upon the instrument must be compensated for temperature, because the conductivity of the water increases with a rise, and decreases with a fall in temperature. This compensation is effected by the compound bars of brass and steel shown in the vessel at the right of the figure. These bars are connected with the wheel and disc from which the electrodes are suspended. When the temperature of the water rises, the bars contract, and exert a pull upon the wheel and disc, so that the electrodes are raised slightly in the U-tube. This increases the length of the column of water between the platinum points, and so increases the resistance, or, what amounts to the same thing, lowers the conductivity, in exact proportion to the rise in temperature. Similarly, a fall in temperature lowers the electrodes, and decreases the resistance by shortening the column of water. In this way the conductivity of the water remains constant so far as temperature is concerned, and it varies only with the saltness of the water. Under ordinary conditions a considerable decrease in the salinity of the water indicates the existence of ice in the near neighbourhood, but the geographical position of the ship has to be taken into account. Rivers such as the St. Lawrence pour vast quantities of fresh water into the ocean, and the resulting decrease in the saltness of the water within a considerable radius of the mouth of the river must be allowed for. A “FLYING TRAIN”