128 MARION" EXPEDITION" TO DAVIS STRAIT AND BAFFUST BAY 



the presence of pack ice, but rarely, if ever, seen in the case of an 

 iceberg. 



Observations which have been carried out on the reflection of sound 

 waves from icebergs emphasize the uncertainty of obtaining echoes. 

 If the sides are steep or perpendicular, an echo will probably be 

 heard from some directions but not from others, depending whether 

 or not the face presented to the sound is normal or oblique. Echoes 

 have been obtained from icebergs at a maximum distance of 1,000 

 yards under favorable conditions, but as a rule are inaudible at a 

 quarter of a mile. A fantastically shaped berg with steep sides and 

 a high peak toward the center was found to give echoes and reechoes, 

 but on another occasion a similar-shaped berg gave none. An echo 



An iceberg in the Fog Off the Grand Bank 



Figure 85. — May 31. 19-8, an iceberg in a light, low fog. This ice was visible from 

 the crow's nest of the ice patrol ship when 3 miles distant, but was not visible 

 less than 1 mile from the ship's bridge. Grave menaces to the trans-Atlantic 

 steamships are the icebergs which are often hidden in fogs of the Grand Banks 

 for weeks at a time. Bergs in light fogs and hazy weather often appear much 

 larger than they actually are. (Photograph by Lieut. Commander N. G. Ricketts.) 



around bergs during a fog may or may not come from the ice, as fog 

 banks under certain conditions are known to be good sound re- 

 flectors.^" 



CURRENT AND WIND CONTROL DRIFT OF ICEBERGS 



The first question to be considered in this connection is the relative 

 importance of the two agencies, viz. the gradient current and the 

 wind, which are responsible for the drift of icebergs. The propor- 

 tion in which these factors combine may vary greatly and is de- 

 pendent upon (a) the proportions above and below the surface of 

 the sea at which a berg floats, (h) the velocity and duration of the 

 wind, and (c) the velocity and depth of the gradient current. 



""Johnston (1913, p. 22) relates a case where the Coast Guard cutter Seneca, lying be- 

 tween a berg and a growler, blew her whistle and received au echo from the growler but 

 none from the berg. 



