26 : The Atlantic 



30th meridian west longitude. Along this line from the mouth of the 

 Indigirka River in Siberia to the Weddell Sea area in the Antarctic, the 

 Atlantic extends more than halfway around the globe. 



These measures have for the most part concerned the surface of the 

 sea, the area that has been of the greatest practical importance to 

 man. To become familiar with the nature and behavior of the upper 

 layers of the ocean and to have taken accurate sounding to the 100 

 fathom mark (600 feet) was an accomplishment that took centuries 

 of human effort. The importance of the 100 fathom mark is that the 

 sharp break between the shelf on which a continent sits and the steep 

 slope in the deeper ocean usually is found within this limit and fre- 

 quent soundings within this mark are of greatest service to the navi- 

 gator. 



Until 1920 deep-sea soundings were very difficult to make and only 

 some thousands of soundings existed for each ocean where hundreds 

 of thousands were needed. It was thought, therefore, that the conti- 

 nents sat on their respective shelves; that somewhere between the 80 

 and 100 fathom line the shelf itself terminated sharply and that the 

 continental slope began which descended in a steep but regular fash- 

 ion to the ocean floor; the ocean floor was flat, monotonous, slightly 

 undulating. Differences in depth in various oceans were recognized 

 and some deep areas had been reported and to these were assigned the 

 name "abyss," but changes in depth were gradual. 



The practical development and application of sonic depth finding 

 around 1920 changed all this. The continental slopes were discovered 

 to be cut by "gorges" and "canyons," the ocean floor to be broken 

 into "ridges," "basins," "trenches," etc. In limited areas mountains 

 have been revealed that are quite as steep and rugged as mountains 

 ashore. Millions of soundings are now available, in fact they accumu- 

 late more rapidly than they can be checked and plotted. Day by day 

 oceanographers in private agencies and government departments are 

 busy exploring and mapping this new, remote and unseen world be- 

 neath the ocean surface. 



Though our chief concern is with the upper layers of the ocean we 

 should be aware that the world beneath the waves is not as inactive 

 or remote as we sometimes suppose. It is an integral part of the entire 

 ocean and even the shapes found in the bottom of sea or ocean may 

 influence behavior at the surface. We have neither time nor space to 

 explore this matter in detail but even a few examples will make us 

 more familiar with the whole Atlantic. 



To begin with a simple matter our surface view of the area of a 



