194 CHALLENGER 



ment of the 'Glass Bucket' had found the flaw in their system 

 but the ship had had a good run. 



Passing through the Panama Canal in October, 1 9^0, Challenger 

 sailed northward in the Pacific towards San Diego. For the last 

 five days' steaming before reaching San Diego the 'deep scattering 

 layer', as it is called, was seen at dawn and dusk on the echo 

 sounding machine. This is a common phenomenon in the oceans 

 and appears on the echo sounder as a false sea-bed echo. The 

 layer spends the day at about 200 fathoms, ascending steadily 

 through the water at sunset until it has reached a depth of 20 

 fathoms or so, where it spends the night, and whence it returns 

 again just before sunrise to 200 fathoms. 



The layer may be caused by countless millions of tiny animal 

 plankton forms, hard-shelled, and known as euphausiids. These 

 animals, which, with many similar types, abound in the oceans, 

 are known to migrate towards the sea surface at nightfall. But 

 the echoes seen on the sounding machine may be coming from 

 vast numbers of fish which live in the deep ocean and are feeding 

 on the migrating plankton : such fish have air bladders aind may be 

 a cause of the echoes being reflected. This may be the answer, 

 possibly combined with other, unthought-of, factors, and as one 

 watches the unbroken layer hour after hour on the sounding 

 machine one thinks of the wealth of life within the oceans. 



At San Diego a very brief visit was paid to the Scripps Institute 

 of Oceanography, a department of the University of California. 

 From here, under the leadership of Dr. Revelle, the Director, 

 four great oceanographic expeditions have sailed out into the 

 Pacific since the close of the last war. 



At Scripps the recent scattering layer records were being 

 discussed and one of their biologists investigating the cause of this 

 layer suggested that Challenger should take one of his depressors, 

 which, attached to a plankton net, would keep it at a constant 

 depth as the net was towed through the layer to capture specimens 

 of the plankton. The depressor was of brass, heavy, and of an 

 intriguing curvaceous shape. The inventor said that the inspira- 

 tion had come to him when contemplating his attractive secretary. 

 The party from Challenger, looking across the room to where she 

 was sitting, saw what he meant. 



