i895. RESULTS OF '^ CHALLENGER" EXPEDITION. 15 



150° W. long, to the American coasts, and there are elsewhere serious 

 gaps in the observations both of temperature and salinity. But from 

 the nature of the case we must expect the growth of our knowledge 

 of the depths of the great oceans to be slow ; and it is, therefore, all 

 the more satisfactory that it is now possible to obtain such a general 

 view of average conditions as will enable us to appreciate the 

 bearings of detailed studies over restricted areas. 



In meteorological work, where temperatures below freezing are 

 of common occurrence, and where isobaric and isothermal charts are 

 frequently used for purely practical and unscientific purposes, there 

 is much to be said for the retention of the Fahrenheit scale ; but in 

 an inquiry of this kind, which appeals almost exclusively to scientific 

 specialists, it seems, indeed, a pity that the Centigrade scale was not 

 adopted. Not only is it impossible to compare or combine Dr. 

 Buchan's results with the observations of the majority of oceano- 

 graphers and marine zoologists, but the transformation to the Centi- 

 grade scale must be effected before a specific gravity calculation can 

 be made with the tables published in another part of the " Challenger" 

 Report. It is further to be regretted that specific gravities are 

 calculated for a temperature of 60° Fahr., and referred to pure 

 water at 39'2° Fahr., since, in this case, the temperature diffi- 

 culty could have been altogether avoided by expressing salinities 

 in "pro milles " in the usual way. In the chart of mean annual 

 surface temperature, areas above 60° Fahr. are coloured red, 

 while colder areas are coloured blue, and the land is filled in with 

 black tints ; the general distribution of temperature being thus made 

 abundantly clear at a glance. At the depths down to 1,500 fathoms 

 a different method is followed, — the sea area is tinted according to 

 depth, and only the isothermal lines are drawn, these being blue or 

 red in each map according as they represent a temperature below or 

 above the average of all the observations employed in drawing them. 

 We could have wished that it had been possible to combine the 

 undoubted advantage of representing the relation of the temperature 

 at any part of a horizontal plane to the mean for the whole of that 

 plane, with a uniform scheme of colour like that given on the surface 

 chart ; for the change of colouring in each map makes it difficult to 

 form a clear mental picture of the vertical distribution between one 

 plane and another. Further, the fact that the contour-colouring in 

 all the maps is identical leads to confusion between open areas and 

 closed basins, especially as in many cases the lines are drawn free- 

 hand so as to show the temperature at a depth which, according to 

 the contour map, does not exist. This difficulty could have been got 

 over by representing, for example, the whole area within 500 fathoms of 

 the surface as land, in the maps of planes below 500 fathoms, and so on. 



Perhaps the most interesting feature of the map of mean specific 

 gravity at the surface is the relation of the areas of highest salinity to 

 the great ocean anticyclones. The maximum evaporation occurs, as 



