58 



MARIOX EXPEDITION TO DAVIS STRAIT AND BAFFUST BAY 



structed on the assumption that the speed of sound in the water 

 column is always 820 fathoms a second. This, as a matter of fact, 

 never was the truth throughout the entire expedition, it being less 

 than this value in every case. Therefore, every fathometer sounding 

 recorded w^as, theoretically at least, somewhat too great. 



The speed of sound in sea water varies principally according to 

 the salinity, temperature, and pressure. The soundings themselves, 

 by giving at least the approximate depth, furnished a clue regarding 

 the latter factor, and the first two could be determined with con- 

 siderable precision from the salinity and temperature values ob- 

 served at the 191 oceanographic stations which the Marion occupied. 

 For complete information regarding this station data, the reader is 

 referred to United States Coast Guard Bulletin 19, part 2, The 

 Oceanography of Baffin Bay and Davis Strait. This pamphlet is 

 not yet published, but it will jjrobably be distributed in 1932. 



The first step in correcting the fathometer soundings was to deter- 

 mine the speed of sound at each station. This speed was obtained, 

 after averaging the salinities and temperatures for each 200-fathom 

 water layer at each station, from the tables on pages 160 and 161 

 of Hydrographic Manual, J. H. HaAvley, Special Publication No. 

 143 of the United States Coast and Geodetic Survey, 1928. The sta- 

 tions were occupied to various meter levels, but it was possible to plot 

 the salinities and temperatures to a meter scale, graphically convert 

 to fathoms, and average conditions by 200-fathom layers in every 

 case by inspection without any important error. 



The United States Hydrographic Office, the Carnegie Institution 

 of Washington, and the United States Coast and Geodetic Survey, 

 all assisted the writer with information and with advice on how 

 to attack the problem or determining the speed of sound in the 

 water columns of Davis Strait. The United States Coast and 

 Geodetic Survey went over the preliminary soundings coi-rection 

 Avork in several of its stages, independently checking a large number 

 of the speed of sound values shown on Figure 46. Because of this 

 checking, the writer feels much more confident regarding the accu- 

 racy and value of this chart than otherwise would be the case. 



After the first speed of sound chart was constructed, all the 

 MaHon''s oifshore soundings and all the speed of sound values were 

 plotted on the Mercator projection on two large rolls of tracing 

 paper, ruled up on a scale of 4 inches to the degree of longitude. 

 The various soundings were grouped on the ncAV chart with different 

 speeds of sound varying by 2 fathoms per second. Corrections were 

 then made in accordance with the table given below : 



Speed of sound i)i water column at sounding spot in fathoms per second and 

 percentage correction to be applied to fathometer readings 



