40 bulletin: museum of comparative zoology. 



and the depth can be calculated from the angle of the rope as observed 

 by the dredging quadrant (Tanner, 1897) and the length outboard. 



The need for a high degree of accuracy in oceanographic research 

 has been emphasized by Helland-Hansen and Nansen, (1909) who have 

 shown that in waters as comparatively well known as those of the 

 North Sea and the Norwegian Sea, inaccurate salinity observations 

 are worse than none, as they give a wholly misleading idea of the water- 

 circulation. The same is true also of temperature readings, especially 

 at great depths. But in a preliminary^ survey of a field, so little known 

 as the Gulf of Maine, the same high degree of accuracy is not so essen- 

 tial, for any information which can be relied on as approximately 

 correct is of value. Nevertheless, the more accurate the determina- 

 tions the better, for the sake of future comparisons. In any case, it is 

 essential that the probable limits of error of the obser^'ations for both 

 salinity and temperature should be clearly stated, and constantly 

 borne in mind in all discussions. 



In the determinations for salinity we are pro\'ided with a perfectly 

 satisfactory water-bottle; the storage of the samples is not open to 

 any apparent criticism, and our burette and pipette are of the best. 

 The instrumental error, therefore, must be very small indeed; and 

 there remains only what we call the personal error of the observer. 

 Unfortunately no trained chemist was available for the titrations;^ 

 and I must confess that I have found the determination of the precise 

 point at which the color changes from yellow to orange a difficult one. 

 Nevertheless, as every sample was titrated twice, some of them three 

 or four times, as the standard water could be relied upon, and as an 

 actual test (p. 62) has shown that repeated tests of the same samples 

 did not differ by more than .01 of salinity, I believe that the results 

 arrived at are reliable considerably within the requirements of the 

 International Committee for the exploration of the sea, i. e., ^.05 

 of salinity, probably to =•= .02 of salinity. 



In the case of temperature, a very high standard of accuracy could 

 not be expected from the instruments which we used. Our deep-sea 

 thermometers were graduated only to l.°F; and the graduations are 

 so rough that we found it impossible to rely on estimation closer than 

 .2°F, though the readings were taken with a reading lens, and estima- 

 tion to .1°F was constantly attempted. We must also consider the 

 possibility of error resulting from not knowing precisely the tempera- 

 ture of the detached mercury thi'ead when read, though the table of 

 correction shows that an error here of 5°F, at the usual air temperature 

 of 55°-70°F would make a difference of only about . 1°F in the reading. 



