506 



HERSEY AND BACKUS 



[chap. 13 



H 



r 



3600 

 3700 

 3800 

 3900 

 4000 



Fig. 7. The daytime appearance of deep scattering layers observed by a 12 kc/s echo- 

 sounder in the eastern Pacific off northern Chile. The oblique line extending from the 

 top of the record represents the lowering of a bottom corer from the ship. 



record of a layer during its migration. This is undoubtedly due to changes in the 

 scattering spectrum of the responsible organisms as the hydrostatic pressure 

 on them changes ; this is discussed elsewhere in this chapter. 



The average migration rates of elements in the 500-m layer are much faster 

 (about 7.5 m/min) than those of the 240-m layer (about 4.5 m/min). The rate 

 of ascent of an element shows little difference from its rate of descent. The 



Fig. 8. The sunset ascent of deep scattering layers recorded bj' a 12 kc/s echo-sounder in 

 the western North Atlantic near 40° 30'N, 50°W. 



