SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 21 



portion as to ensure the degree of accuracy above indicated, and provision for extending 

 its range of usefulness to sea-waters of all specific gravities should be made by suitable 

 alterations of its weight. Considerations of stability suggested attaching the accessory 

 weights necessary for this purpose to the lower extremity of the hydrometer. But this 

 would involve their being immersed in the liquid the specific gravity of which was to 

 be determined, and was therefore inadmissible. The only alternative was to attach them 

 to the upper extremity of the stem. A length of 10 centimetres of the stem was 

 graduated into millimetres, and the external diameter of the stem was such that its 

 graduated portion displaced rather less than 1 gram of distilled water. The body of 

 the instrument was constructed so as to have a volume of approximately 160 cubic 

 centimetres. The hydrometer was ballasted with mercury, so as to fioat in distilled 

 water of ordinary temperature with the whole of the graduated part of the stem exposed. 

 The system of accessory weights designed for increasing the range of the hydrometer 

 included, as first weight, a small brass table which fitted on to the top of the stem. Its 

 weight was designed so that, if the hydrometer alone floated at the lowest division of 

 the scale in a particular water, and the table was then affixed to the top of the stem, 

 the hydrometer would sink until it floated at a division near the top of the scale in the 

 same water. Of the further weights, the first of the series was a mass of brass of about 

 the same weight as the table. When the hydrometer carrying the table on the stem 

 floated at the lowest division of the scale in a particular water, then, by placing the 

 further weight on the table, the hydrometer sank until it floated at a division 

 near the top of the scale. The weight of the next weight of the series was made 

 approximately double that of the table, so that when the hydrometer, loaded with 

 the table and the previous weight, floated at the lowest division on the scale in a 

 particular water, and the previous weight was replaced on the table by the present 

 one, the hydrometer floated in the same water at a division near the top of the 

 scale ; and so on. 



S 6. The series of weights was carried so far that waters of all densities from that of 

 distilled water to that of water more dense than that of the Red Sea could be determined 

 witli the same hydrometer. 



The accessory weights form roughly an arithmetical series, the common diff"erence 

 of which is equal to the first term, namely, that of the little table to be placed on the 

 top of the stem and to carry the other weights, when required. As produced, the 

 weights fulfilled all the conditions demanded of them, and all that it was necessary to 

 know was the exact weight of each, and this was determined. 



From the design of the system of accessory weights, it will be seen that provision 

 was made for single observations of specific gravity. Duplicate observations were 

 possible only in cases where the salinity and temperature of the water combined to 

 produce such a specific gravity that it could be observed with one of the sets of weights 

 near the lowest division of the scale. In that case its specific gravity could be obtained 

 also with the next higher weight at a division near the top of the scale, because the 



