SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 23 



Weight No. is the small brass table which can be affixed to the top of the 

 stem, and on it any further weight that might be required was placed. The distinctive 

 number of the hydrometer which was used for all the determinations made in the 

 Challenger was 0. In the tabulated results the combination used is indicated in 

 the column headed " Number of Hydrometer." Thus, OOV means that Hydro- 

 meter No. 0, table No. 0, and weight No. V. were used. The combinations almost 

 exclusively used were OOlV and OOV, which weighed 1642633 and 165H98 grams 

 respectively. 



In this memoir we make no use of the volume of the hydrometer, because in 

 all the experiments the temperature is a constant, and we obtain directly the dis- 

 placement, that is, the weight of distilled water displaced by the same volume of 

 saline solution, both being at the same temperature, from which we obtain directly 

 the specific gravity of the solution at that temperature, referred to that of distilled 

 water at the same temperature as unity. This result is arrived at from the two 

 observations alone, and is independent of the work of others. 



As it was certain that during the voyage of the Challenger the specific gravity 

 of the sea-water would have to be observed at many diff"erent temperatures, it was 

 convenient, after having determined the displacement of the hydrometer in distilled 

 water at diff"erent temperatures, to express the result in terms of the volume of the 

 displacing hydrometer in standard cubic centimetres, but the difference from the 

 later practice is only in the form of expression. 



§ 7. In order to obtain all the precision of which the hydrometric method is 

 capable, the temperature of the water must remain perfectly constant while the 

 hydrometer is floating in it, and the temperature of the hydrometer must be the 

 same as that of the water before it is immersed in it. In ordinary work on shore 

 and in our latitudes this is the condition which it is most difficult to realise. In 

 the Challenger it provided itself Nearly three out of the three and a half years 

 that the voyage lasted were spent between latitudes 40° N. and 40° S. Here the 

 temperature of the air is relatively high, but its diurnal variation is very slight. 

 Moreover, the Challenger was a wooden ship, and the laboratory was lighted and 

 ventilated by a large main-deck gun-port, the result of which was that, especially 

 in the tropics, the temperature of the air was almost constant, day and night. 



The temperature of the surface water was usually slightly higher than that 

 of the air, but only by a fraction of a degree, so that its specific gravity could be 

 determined immediately after collection. Samples of water brought up from the 

 bottom and the inferior depths arrived on board having a temperature much lower 

 than that of the air, and it was impossible, even if it had been convenient, to 

 proceed at once to the determinations of their specific gravity. A case containing 

 eight large stoppered bottles was kept in the laboratory for the purpose of receiving 

 these samples as they arrived, and they were kept in the laboratory until the next 

 day, and their specific gravities were then determined one after the other. The 



