74 



is very soon clear to observers on the patrol ships, control to a great 

 extent the immediate future course of weather in the ice-patrol area. 



DEPTH SURVEY CARRIED OUT BY THE SONIC METHOD 



Throughout the 1929 ice-patrol season both the Tampa and Modoc 

 were equipped with commercial instruments for determining the 

 depth of the water by sonic means. It was usually possible to use 

 these instruments successfully so long as the ships remained inside 

 the 1,400-fathom curve. On smooth days when the sets were work- 

 ing especially well they could be used in water up to about 2,000 

 fathoms deep. Whenever echoes could be obtained from the sea bot- 

 tom frequent soundings were taken and recorded. 



One hundred and ninety-one values that were obtained when the 

 vessels' positions were well fixed by sights have been corrected for 

 certain errors due to actual conditions of salinity, temperature, and 

 pressure in the water column and forwarded to the United States 

 Hydrographic Office and to the United States Coast and Geodetic 

 Survey for use on charts of the North Atlantic Ocean. Altogether 

 many times 191 values were recorded, but the great majority of sound- 

 ings, though useful for immediate navigational purposes, were taken 

 when the exact geographical position was in some doubt due to such 

 things as abnormal refraction, cloudiness, darkness, and fog. The 

 depths obtained when the position was uncertain were without ex- 

 ception discarded so far as giving them consideration for hydro- 

 graphic use was concerned. The area where the patrol vessels cruised 

 in 1929 has been rather well sounded out, and for this reason noth- 

 ing but the best work of the season was considered worth keeping. 



Two navigators worked out the different positions of the ships 

 independently for check purposes, so the locations listed with the 

 sounding values saved are based on double work and are believed to 

 be as nearly correct as such values can be on board a ship on ice- 

 patrol duty in the Grand Banks region. Of course some of the posi- 

 tions are closer to being right than others. The radius of error prob- 

 ably varies from next to nothing at all up to a maximum of about 

 10,000 yards. 



