INTRODUCTION. 



More than one hundred and fifty years ago 

 Thomas Gray, in his famous " Elegy Written in 

 a Country Churchyard," penned these lines: 



" Full many a gem of purest rays serene, 

 The dark, unfathomed caves of ocean bear." 



Until the middle of the nineteenth century very 

 little was known of the immense ocean depths. 

 Speculation and imagination had been rife with 

 regard to the life and conditions below the surface 

 of the ocean, but little definite knowledge had as 

 yet been gleaned. 



Towards the close of the year 1872, H.M.S. 

 "Challenger" sailed from Portsmouth on a voyage 

 round the world, which lasted for three and a 

 half years. The main object of this cruise was 

 to investigate the physical conditions and natural 

 history of the deep sea in various parts of the 

 world. The ship was equipped with special 

 apparatus for sounding and dredging purposes, 

 for obtaining specimens of sea-water and its 



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