BIOLOGICAL SURVEY OF WOODS HOLE AND VICINITY. 



53 



some cases they are accurate only to the third decimal place. Those familiar with 

 recent hydrographic studies w-ill perhaps regard such figures as too rough approxima- 

 tions to have any scientific value. This would doubtless be true if we had to do with wide 

 expanses of the sea, containing fairly permanent currents or strata of water, the limits of 

 which could only be ascertained by determining slight differences of salinity. But in the 

 inclosed bays and sounds of our region the continual intermixture of the waters resulting 

 from tides and winds would render unlikely any constant stratification on the basis of 

 salinity, and it is certain that rapid variations occur within the same area. As was the 

 case with the temperature records already discussed, a series of determinations having 

 no reference to the phase of the tide are open to rather serious objections. But it would 

 be practically impossible to make such a series simultaneously throughout so large an 

 area, and almost equally difficult to make each of them at a corresponding phase of the 

 tide. For these reasons, therefore, only the larger differences of water density, such as 

 are indicated by figures in the third decimal place, seem to be of interest in attempting 

 any correlation between this factor and the distribution of our local marine animals 

 and plants. And it will be found later that, so far as our dredging records are concerned, 

 even the greatest extremes of salinity which are recorded by us have little or no effect 

 in limiting the distribution of most of the species. This statement, of course, is only 

 intended to apply to the fauna taken by the dredge. Great numbers of littoral or 

 shallow-water organisms, here as elsewhere, obviously thrive best in brackish water or 

 at least in somewhat diluted sea water. The salt marshes and the estuaries, indeed, 

 are largely populated by a fauna of their own. 



The figures for density are given in the same tables (i-S) as those for temperature. 

 From the density figures those for salinity proper, or percentage of salts, may readily 

 be obtained from the table offered by Pettersson (1894, p. 298). The following equiva- 

 lents have been computed for such degrees of density as are to be found in Buzzards 

 Bay and Vineyard Sound. They represent the percentages of salt by weight in a given 

 quantity of sea water: 



The dift"erences of salinity, in relation to locality and season, are represented upon 

 charts 215 to 218. Several facts of importance are to be derived from these tables 

 and charts. 



(i) Even the highest figure recorded here (1.0244) is considerably lower than that 

 found throughout the north Atlantic at great distances from land, where a specific 

 gra\ity of 1.0270 to 1.0280 prevails. 



(2) The greatest extremes to be found among our determinations are 1.02 12 and 

 1.0244, representing a difference of about 15 per cent in salinity. 



(3) The average surface density (1.02337) is lower than the average density at the 

 bottom (1.02349). This difference is more marked in the Bay than in the Sound. It is 



