ECHO-SOUNDING AND H YDROGR AP HIC AL STUDIES 



33 



Four sections of the bottom of the 

 Philippine Trench at distances of 

 a little over lOO kilometres. The 

 bottom of each block is lo kilo- 

 metres deep. 



overclouded skies which precluded astronomical observations for days on 

 end; and as the current often ran at a speed of one or two knots, mostly 

 in a westerly or south-westerly direction but ver\- irregularly, many con- 

 jectured positions are rather uncertain and echo curves obtained on these 

 days can only be supplementan." to more accurately determined sections. 



The large number of sections which we sounded across the trench pro- 

 vided us with so much information about the course of certain charac- 

 teristic features that the position of many sections could actually be 

 checked by the form of the echo curves. 



The trench extends from about latitude 13° 30' N to about 4' 30' N 

 — that is, not less than 540 nautical miles ^ 1,000 kilometres^ — at depths 

 beyond 7,500 metres. But the actual deep on which research was mainly 

 concentrated is the stretch of about 300 kilometres along the northern 

 tip of Mindanao. The above diagram shows in schematic form some 

 typical sections of the trench at intervals of rather more than 100 kilo- 

 metres. From the coast the floor shelves more or less steadilv down to a 



