58 



United States Coast and Geodetic Survey, for their information, in 

 connection with improving the soundings data on the North Atlantic 

 Ocean charts concerned. 



As the instruments were a new type never before used on ice 

 patrol, the following remarks are given regarding their action during 

 the different partol cruises. These have been taken in substance 

 from the cruise reports of the commanding officers on the several 

 patrols. 



At the beginning of the first cruise only up to 200 fathoms could be 

 sounded with the sonic depth finder on the Mojare. Even this was 

 of value in conducting the search for ice, being sufficient to give instant 

 notice when to change course at such times as when approaching or 

 leaving the 100-fathom curve of the Banks. As the patrol went on 

 the depth that could be sounded increased to a maximum of 1,300 

 fathoms. It was believed that this was due to smoother water which 

 lessened the so-ealled water noises from whitecaps, and to increasing 

 experience on the part of the operators, for the machine itself had 

 practically no adjustments made to it. It was very encouraging 

 when soundings could be taken in the deep water because of the com- 

 parative scarcity of depth values on the chart in the area being 

 searched. The greatest care was taken to keep track of the ship's 

 position at all times in order that the locations of the soundings might 

 be accurate. , 



The second cruise was started ^\^th a systematic gathering of fath- 

 ometer depths on the Modoc. The officers of the deck, by taking 

 soundings every 15 minutes, quickly gained experience in deep water 

 depth sounding and obtained a number of particularly valuable 

 records. Due to the small amount of wave motion and consequent 

 quietness on April 8, audible echoes Avere obtained in depths up to 

 2,000 fathoms. On April 10 the apparatus broke down and could not 

 be adjusted until engineers from the maker boarded the vessel at 

 Halifax after the end of the cruise. 



It was routine during the third cruise for the officers of the deck 

 on the Mojare to get the depth by the echo method every 15 minutes 

 while cruising and every half hour while drifting. Only occasionally 

 was there difficulty, as when the signals were weak in the phones. 

 In all 1,700 successful echo soundings were recorded in the rough book 

 on the bridge during the cruise. Of these 400 were preserved for 

 use in checking the North Atlantic charts. The dift'erence was due to 

 the fixed policy of rejecting for hydrographic use all soundings 

 made when the position of the ship was in doubt due to lack of good 

 sights. 



Tlie fourth cruise produced only 22 values of depth for the smooth 

 record. The high percentage of bad visibility, coupled with inability 

 to hear the fathometer echoes in the extremely deep water, where 



