CHAPTER IV. 



THE TEMPERATURE SECTIONS SURVEYED BY H.M.S. 

 "CHALLENGER" IN THE ATLANTIC. 



From Teneriffe to Sombrero and St. Thomas — From St. Thomas to Halifax — Between 

 Cape May, U.S., and Madeira — From Madeira to Tristan d'Acunha — Between 

 Cape Palmas and Cape S. Roque — Between Cape S. Roque and Tristan 

 d'Acunha — From the Falkland Islands to the Cape of Good Hope. 



The Oceanic Temperature Sections Surveyed by H.M.S. 

 " Challenger." — The accompanying diagrams and tables, 

 especially constructed by the author for this essay, embody the 

 principal results of the sounding operations carried on by the 

 officers of H.M.S. " Challenger " during her cruise round the 

 world between December, 1872, and May, 1876. 



The isotherms of 2°.5, 5 , io°, 15 , 20 , and 25 C, have been 

 selected, partly as affording a sufficiently correct representation 

 of the distribution of temperature in the different oceanic sections 

 which have been explored, partly because the above degrees of 

 the Centigrade scale correspond with even numbers of the 

 Fahrenheit scale, namely, 36°.$, 41 , 50 , 59 , 68°, and yy°. The 

 intervening isotherms are, as a general rule, symmetrically 

 arranged between these limits. As the temperature of io° C. 

 (50 F.) fairly marks the point which divides what may be called 

 warm water from cold water, the strata of a temperature above 

 io° C. have been coloured red, those below that temperature 

 blue. The strata coloured violet are those in which the tem- 

 perature of the water has been found to remain unchanged down 

 to the bottom (Plates 15 and 16). The yellow or buff tint 

 indicates where bottom has been reached within 1500 fathoms 

 from the surface. It should also be observed that the scale of 



