34 ECHO-SOUNDING AND HYDROGRAPHICAL STUDIES 



depth of 6,000 — 7000 metres, after which it drops quickly, by means of 

 steep cHffs of varying height akernating with terraces, down towards the 

 bottom of the trench at a depth up to about 10 kilometres. 



From the terraces we got clear echoes revealing a gradient of less than 

 one in five or one in six (some 10°) ; but whenever we passed over the 

 steep cliffs there was a total absence of echo, and it was of no avail to 

 reduce speed or stop. Echoes returned just as suddenly, but at a new level, 

 as soon as we were once more over a bottom with a sufficiently gentle 

 slope. The Cape Johnson often lost the echo in the same way and had to 

 send out impulses up to 10 times before getting any response. The pre- 

 sumed reason was that the bed was too soft to throw off an echo. The rea- 

 son suggested by our experience is that in getting an echo from the tenth 

 emission when she had not done so from the previous ones she was by 

 then located over a sufficiently level floor. We always got a clear echo 

 from a le\'el bed, regardless of the softness of the bottom material brought 

 up by our gear. 



The lowermost terrace on the western incline of the trench was frequently 

 so near to the bottom that parts of the two levels fell within range of the 

 echo-sounder simultaneously (the range being the circle of horizontal 

 floor from which it is possible for the ship to obtain an echo). As a result 

 we got two series of dots on the echogram, showing two depths with a 

 difference of perhaps 200 metres, a picture so characteristic that we were 

 able to take it as a sign that the current was carrying us too far west. The 

 eastern incline, on the other hand, often had such high cliffs that for a 

 time we got no echo at all. Since all work with trawl or grab is confined 

 to the flat floor of the trench, and this is mostly \ery narrow — down to 

 as little as 750 metres — these "cliff echoes" are very valuable; indeed, 

 are the only means by which it is possible to navigate with sufficient 

 accuracy. 



The eastern slope is more irregular than the western slope, not only in 

 regard to gradient but also in the course which it takes along the trench, 

 and in some places there seem to be promontories, in others small plains 10 

 kilometres in depth. It was near one such plain that the Cape Johnson found 

 the gi"eatest depth; namely, 10,497 metres at latitude 10° 27' N. And as 

 we found suitable trawling ground at the same position, we cruised about 

 the area, crossing the indicated position of the Cape Johnson on a num- 

 ber of occasions. We found the lexel floor mostly at a depth of about 

 10,000 metres, and the greatest depth registered on our expedition — 

 10,265 nietres — was at this place, about three and a half nautical miles 

 south of the Cape Joh?ison's position. As the Cape Johnsoits position is 



