52 BULLETIN OF THE BUREAU OF FISHERIES. 
miles from Marthas Vineyard. The tides, likewise, are of much greater amplitude 
north of Cape Cod, insuring a far more rapid intermingling of the coastal waters with 
those of the open sea. South of Cape Cod there is an extensive area of shoal water, 
much of which is pretty definitely bounded off from the open ocean. Reference has 
already been made to the occurrence of a net westerly tidal movement through Vine- 
yard Sound. This implies, of course, that the latter derives much of its water from 
Nantucket Sound, a broad and on the whole very shallow area of sea, pretty well shut 
in by land and by shoals. 
5. SALINITY. 
Salinity or, more properly, density determinations were made along with those for 
temperature. The Sigsbee water cup was employed for obtaining samples from the 
bottom, while the surface water was merely drawn up in a pail. The salinometers 
employed were of the Hilgard pattern and were previously tested by the Bureau of 
Standards. Great care was taken to prevent the soiling of the stem by the hands, 
which was found to exert a marked effect upon the level reached by the instrument. 
A bottle of caustic soda solution, or a mixture of sulphuric acid and potassium bichro- 
mate, was kept at hand, and used from time to time for cleaning the stem. It was 
found more practicable to read from the summit of the meniscus, or cone of fluid sur- 
rounding the salinometer stem, than to read from the actual water level. The value 
of the meniscus in terms of the scale was later determined. Since the temperature of 
the water is an all important factor in determining its specific gravity, as referred to 
distilled water at maximum density, careful record was kept of the water temperature 
at the time of taking the reading for density. Knowing these two factors, reduction 
was easily accomplished by the aid of a table furnished by the Bureau of Standards.” 
The figures, as presented, represent the specific gravities which would have been 
obtained had the water samples in all cases been at a temperature of 15° C. Thus each 
figure represents the relative weight of a given sample at 15° C. compared with an equal 
volume of distilled water at 4° C. The density of a solution depends, of course, upon 
two factors, its temperature and its concentration. Having eliminated all differences 
due to the former factor, the figures,here given represent the concentration, i. e., the 
salinity of the water. 
The density readings here recorded were in nearly all cases made aboard ship. 
More precise determinations would of course have been possible if the water samples 
had been bottled and brought back to the laboratory where the ship’s motion would not 
have disturbed the observations.? And our results would have been still more precise 
had we resorted to the method of titration with nitrate of silver, as employed in recent 
hydrographic studies.° The latter method has, however, been used by us as a check 
upon our specific gravity determinations, and the results of the two accord so well on 
the whole (see p. 54) that the figures here presented are probably exact enough to meet 
the demands of the present work. Our figures for density are recorded to the fourth 
decimal place. From comparison with the chlorine tests it seems likely that in 
@ Various tables of this sort have been published; e. g., Libbey, 1891, p. 397; Tanner, 1897, p. 337- 
b In five cases, in which this was done, and the results of the two independent determinations were compared, a mean differ- 
ence of 0.00024 was fotnd; i. e., the error affected only the fourth decimal place, or last one considered in making the reading. 
¢See Pettersson, 1894, p. 296; also account of International Conference for the Exploration of the Sea, in Journal of the Marine 
Biological Association, vol. vI, pp. ror—rr4. 
