Preface. 



the numerous conditions which must more or less determine the 

 direction, velocity, volume, and persistency of oceanic currents — 

 such, for example, as the unequal distribution of solar heat over 

 the surface of our planet, the agency of the winds or atmospheric 

 currents, the difference of temperature, specific gravity and 

 chemical composition of the water, the direction of the coast- 

 lines, the distribution of land and water in general, the 

 configuration of the sea-bottom, &c. It remains to the 

 student to ascertain, with the help of the observations at his 

 disposal, the part which is to be assigned to each of these 

 conditions in the great phenomenon of oceanic circulation, 

 considered as the final product of various causes, all acting and 

 reacting upon each other. 



Amongst the principal efforts in the domain of deep sea 

 exploration, we have the labours of the officers of the United 

 States Coast Survey along the course of the Gulf Stream 

 (1845 to 1859), the soundings of the U.S.S. " Mercury" between 

 Barbadoes and Sierra Leone in 1871, the observations made on 

 board H.M.S. " Lightning" and H.M.S. "Porcupine" in the 

 seas extending from the Faeroe Islands to the Mediterranean 

 (1 868-1 870), culminating finally in the two voyages of circum- 

 navigation made by H.M.S. "Challenger" (1872-1876) and the 

 German frigate "Gazelle" (1874-18 76). The extensive series 

 of soundings for which we are indebted to these expeditions 

 has received further additions through the operations of H.M.S. 

 " Valorous " between the English Channel and Davis Strait in 

 1875, of the Norwegian ship "Voringen" between Norway and 

 Iceland, of the U.S.S. " Gettysburg" in the North Atlantic 

 in 1876, the U.S.S. "Tuscarora" in the Pacific Ocean (1874 

 to 1876), and the latest Arctic Expedition under the command 

 of Sir G. S. Nares (1875-1S76). 



In the charts and diagrams which accompany the following 

 pages, I have endeavoured to combine the results of recent 



