1872.] 



' Shearwater ' Scientific Researches. 



581 



of North Africa of any mountain-barrier like that of the x\tlas and other 

 ridges which intervene between the Sahara and the coasts of Morocco and 

 Algeria, and cool down those heated winds which blow from Central Africa 

 without any moderating influence over the low expanse of Tripoli and Egypt. 

 And further, while the surface-temperature of the Western basin is also 

 kept down by the admission of colder water through the Straits of Gibraltar, 

 that of the Eastern basin will be elevated rather than depressed by the dis- 

 charge of the vast body of water brought down by the Nile through the 

 hottest months of the year. And thus it comes to pass that the summer 

 isotherm of 80° which runs through the interior of Northern Africa from 

 Mogador to the Syrtis, thence follows the coast-line between Tunis and 

 Syria, then turns northwards along the coast of Syria, and returns along 

 the coast of Asia Minor, so as to enclose the Levant in a "bight" of 

 high temperature. — Looking, again, to the fact that the follom-tempera- 

 ture was found in the previous year to be somewhat higher, even in the 

 Western basin, to the east of Sardinia, than nearer the Strait of Gibraltar, 

 I think it probable that the permanent bottom-temperature of the whole 

 Eastern portion of the Mediterranean is about 2° higher than that of the 

 Western. The cause of this difference, also, is to be sought in the conditions 

 which affect the surface-temperature ; for, as I showed in my last year's 

 Report (§§ 88-91), the complete separation between the deeper water of 

 the Mediterranean and that of the Atlantic will cause the temperature of 

 the former to be the lowest mean of the locality ; and thus as the winter 

 surface-temperature of the Eastern basin is somewhat above that of the 

 Western, its bottom-termperature also will be higher. A remarkable con- 

 firmation of this view is afforded by the fact already stated respecting the 

 temperature of the Gulf of Suez (§ 11). 



71. It might be anticipated, therefore, that such an excess of tempera- 

 ture in the whole superficial stratum heated by direct insolation would 

 show itself in the Eastern basin, as is represented by Series III. (Table and 

 Diagram) when compared with Series I. and II. But the extraordinary 

 elevation shown in the superficial stratum of No. IV. seems to betoken 

 some special calorifying agency ; for although the difference of about 3|° 

 of Latitude in favour of that Station might account for a general elevation, 

 yet as this does not show itself at all in the sw/ace-temperature, and 

 scarcely at all in the 5o^om-temperature, there seems no reason for its 

 showing itself to such a marked degree in the intermediate temperatures, — 

 especially in those of 20 and 30 fathoms. Now if I was correct in the 

 interpretation which I gave, in my last year's Report (§§90, 91), of the 

 mode in which the Sun's heat penetrates downwards by convection, — the 

 surface-films successively concentrated by evaporation sinking until their 

 excess of density is lost by diffusion, — it is obvious that the longer the 

 continuance of surface-heat, especially if combined with dryness, the deeper 

 would its influence extend, though the surface-temperature might not itself 

 be raised. For, as I then remarked, " the continual repetition of this process 



