32 ECHO-SOUNDING AND HYDROGRAPHICAL STUDIES 



speed of rotation \\as checked se\eral times daily \\ith the aid of a stop 

 watch and a chronometer, and adjusted to gi\"e a maximum margin of 

 error of ±0.2 per cent, the Gal(2thca's soundings may be considered ex- 

 tremely accurate. 



The echo-sounder ^\as in continuous operation both while the ship was 

 under ^\■ay and ^vhen we were working at stations, ^^ith the result that 

 our kno^vledge of the ocean floon; has been increased by cur\es covering 

 many thousands of kilometi^es. ^Vhile in general the registered depths 

 agree Nvith pre\ ioush recorded measurements, \\e also found "nc\\ " sub- 

 marme mountains as well as deeps greater than any before kno^\•n. Oiu' 

 first big disco\ery was a mountain 2,800 metises in height \vhich rises 

 steeply in the Indian Ocean between the Seychelles and Ceylon, from 

 an other^\•ise level bed 4,000 metres deep. Our most exciting operation, 

 however, ^^•as the sounding of the great troughs in the Pacific, notably 

 that of the Philippine Trench. 



This, Nvhich at the time of our researches ^\•as regarded as the deepest 

 trench on earth, runs parallel with the eastern side of the Philippines, 

 50 — 100 kilometres from land. Its great depth \\as fii^st obser\cd by the 

 German Planet Expedition in 19 12, when 9,789 metres was measured by 

 wire-sounding. The next sounding ^\•as made by the German cruiser Emden 

 on April 29, 1927, when a depth of 10,800 metres was found at two 

 points a fe^\• nautical miles south of the Planet Deep. For years these two 

 \er\- deep soundings ha\e appeared on charts and in books as the greatest 

 ocean depths. Howexer. doubts ha\e arisen as to their reliability, and it 

 has been pointed out that the Emdcn employed a non-directional beam, 

 so that misleading echoes may ha\ e been obtained from slopes on the bed 

 ^^"ell to the side of the ship's position. The Dutch research ship M'illcbrord 

 Snellius (1930), the U.S. naval vessel Cape Johnson (1944 — 45), and 

 our own Gulathea all made careful soundings round the Emden's position, 

 and all agreed in finding a depth of just o\er 10,000 metres. The greatest 

 depth foimd an)Avherc in the Philippine Trench h\ the Snellius \vas 

 10,160 metres; but in 1945 the Cape Johnson, at a position some 40 

 nautical miles north of that of the Emden, measured 10,497 nietres ^vith 

 a directional echo-sounder, though using headphones. This ^vas the great- 

 est known depths in the Philippine Trench. 



It is possible, howe\cr, that the position of the Cape Johnson when she 

 made her deep sounding was three and a half nautical miles in error. All 

 expeditions, unfortunately, experience the same difficulty of accurately 

 determining the ship's position and keeping that position ^vhile Nsorking 

 at their station. The Galathea had to contend ^vith several periods of 



