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IX. — Specific Gravities and Oceanic Circulation. By Alex. Buchan, M.A., LL.D. 



(With Maps.) 



In the report on Oceanic Circulation, based on the observations made on board H.M.S. 

 " Challenger," and other observations, which was published in the beginning of 1895 as an 

 Appendix in the Summary of Eesults, Second Part Challenger Reports, the specific 

 gravities dealt with were all at the standard temperature of 60° Fahr., the standard 

 density being that of distilled water at 39°'2 (4° C). By this method of treatment the 

 question of the salinity or saltness of the water is approximately stated, unquestionably 

 one of the most important questions affecting the physics of the ocean. But the move- 

 ment of the water, or oceanic circulation, as resulting from different densities, can only 

 be represented by stating, not the specific gravity reduced to the uniform temperature of 

 60°, but the specific gravity at the observed temperature at all points in the ocean at 

 which observations are made. In this paper these specific gravities are viewed in their 

 relations to the circulation of the waters of the ocean. 



In the Challenger Eeport every effort was made to secure that the maps of mean 

 temperature and mean specific gravity of the surface of the ocean were constructed from 

 annual means, since maps thus constructed are altogether indispensable in any discussion 

 of oceanic circulation. This discussion therefore proceeds from data, representing on a 

 map of the earth the mean annual specific gravity of the surface waters at the mean 

 annual temperatures of the surface. Map 1 represents such a map, which has been con- 

 structed in this way : — A table of mean annual temperature for every 10° of longitude 

 and every 5° of latitude was constructed from the map of surface temperature given in 

 the Challenger Report, and a similar table of mean annual specific gravity at the 

 temperature of 60°. * From these two tables another table was constructed, giving the 

 mean specific gravity at the temperature of the surface for the same points of the ocean, 

 numbering in all 640 points. The mean of these 640 specific gravities is T0252, 

 allowance being made for the diminution, with latitude, of the areas of the " squares." 

 Each of the specific gravities was then compared with this general average, the difference 

 entered in its place on the map, and the lines of differences — 0-0010, - 0020, and 0"0030, 

 &c, above and below the average — were thereafter drawn on the map.t 



None of the observations on board the " Vitiaz " were made at greater depths than 

 800 metres or 437 fathoms ; and those on board the " Gazelle " only at the surface, at 

 depths of 50 and 100 fathoms, and at the bottom of the ocean. Hence the observa- 

 tions on board these ships are unfortunately seriously defective as aids in discussing the 

 problem of oceanic circulation. A considerable number of observations, such as those 



* Report, Maps 1 and 2. 



t For sources of information from which the specific gravities have oeen obtained, see Appendix, p. 342. 

 VOL. XXXVIII. PART II. (XO. 9). 2 U 



