CLYDE SEA AREA. 667 



dent in transit by rail to a minimum. In all cases the cork-lined boxes were used for 

 carrying sample-bottles, a layer of straw over the top allowing the lid when tied down to 

 exert a uniform light pressure on the stoppers, and so prevent shaking. 



Salinity and its Measurement. — The careful experiments of many investigators, but 

 particularly of Thorpe and Kucker, # and of DiTTMAR,t have shown that the density of 

 sea water at constant pressure is a function of salinity and temperature alone. Therefore 

 in order to compare the salinity, i.e., the ratio of dissolved solids to the whole sea water, 

 of different samples, it is sufficient to determine the density and the temperature of the 

 water in question. The words density and specific gravity are frequently used as if they 

 were synonymous, and much confusion results from this habit. The density of a sub- 

 stance is the ratio of its mass to its volume, and the numerical value of this ratio, of 

 course, depends on the units of mass and volume employed, e.g., pounds per cubic foot, 

 or grammes per cubic centimetre. To obviate this difficulty, the ratio of the density of 

 the substance under consideration to that of some standard substance has been adopted 

 as the quantity for comparison, and to this ratio the term specific gravity is given. Pure 

 water is universally accepted as the standard substance, but the temperature at which it 

 is held to be the standard varies, and the density of water is a function of its tempera- 

 ture. Hence we find that the term " specific gravity of sea- water " is susceptible of 

 several distinct meanings, and is, indeed, employed in several different ways. 



The maximum density point of pure water (4° C.) is the natural temperature to take 

 as a standard for density, and if this is adopted the specific gravity of sea water may be 

 determined, either at the same or at some other standard temperature, and by means of 

 the tables of dilatation of sea water compiled by the authorities cited, the value of the 

 specific gravity at any temperature may be calculated from the determination at any 

 other temperature. Dittmar represents by 4 S£ the ratio of the density of sea-water at t° 

 to the density of pure water at 4° ; and he adopts as the standard t = 60° F. or 15°'56 C, 

 because that temperature was employed in reducing the " Challenger " results obtained at 

 sea by means of Hubbard's tables. The unit 4 S 15 . 56 will, for the sake of uniformity with 

 the " Challenger " work, be employed as the standard density to which to reduce the 

 experimental results given in this paper. 



The term " specific gravity," as used in the publications of the British Meteorological 

 Office, indicates the ratio of the density of sea water at 15 0, 56 to pure water at 15°"56 ; 

 as used in the reports of the German Hydrographic Office and other continental 

 authorities, it is the ratio of the two densities at 17°*5. Dr J. GibsonJ uses specific 

 gravity to mean the ratio of the two densities at the freezing point, 0° C. ; and this 

 method has special advantages, inasmuch as it admits of rigidly accurate comparison of 

 specific gravities without any theoretical reductions to standard temperature. For 

 accurate work, where a fine balance and abundant time are available, Dr Gibson's method 

 (using a modified Sprengel's pyknometer) is certainly the only one that should be 



* Phil. Trans., cxlvi. (2), p. 405. t Challenger Reports, " Physics and Chemistry," vol. i. 



J Sixth Annual Report of the Fishery Board for Scotland, 1887. 



