SIB CHARLES LYELL. 



237 



incorporated by Sir C. Lyell into his remarks upon the temperature 

 and shape of the bed of the ocean and its living inhabitants. In his 

 chapter on ocean-currents he has also considered the latest-known re- 

 sults of experiments and observations made by Dr. Carpenter, Prof. 

 Wyville Thompson, and Captains Spratt and Nares, upon the currents 

 of the Straits of Gibraltar. The space allotted to this survey is not 

 adequate to a full or critical discussion of the arguments for and against 

 the existence of a permanent indraught. The balance of proof, how- 

 ever, is felt by Sir Charles to support his previously-expressed convic- 

 tion that the inflowing movement is no permanent undercurrent caused 

 by evaporation, but the result of the Mediterranean tide, which, slight 

 as it is, runs alternately from east to west for several hours, its action 

 being found more regular in the depths of the straits, where it is less 

 affected either by winds or by the surface inflow. The difference of no 

 less than twenty degrees between the temperature of the Mediterra- 

 nean and the Atlantic, as well as the difference of four degrees between 

 the deep-sea soundings of the western and central basins of the Medi- 

 terranean and of the Greek Archipelago, is explained by the existence 

 of high submarine crests or barriers of rock bounding the sea to the 

 west, and again dividing it into sections, as shown by the diagram in 

 the present edition. Proceeding to the wider problem of ocean circu- 

 lation arising out of the extreme cold found at great depths both in tem- 

 perate and tropical regions, Sir Charles disputes the notion of these low 

 temperatures being due to mere depth, the Mediterranean soundings 

 of 13,800 feet having failed to reach a degree of cold below 55 Fahr. 

 Yet the soundings taken at Aden, whither the cold water can only 

 come from the Southern Hemisphere, lead to the belief that the whole 

 of the equatorial abysses of the ocean are traversed, in some parts at 

 least, by a continuous mass of water not much above 32 Fahr. That 

 solar heat is in some way or other the primary cause of this great dis- 

 placement, through the change in specific gravity from the cooling of 

 water toward the polar zones, counterbalanced by a return, however 

 slowly, of water from the equator to the Poles, may well take the place of 

 more recondite theories, such as that exploded by Herschel, that the 

 expansion of water by heat in the equatorial zone raises the level of the 

 sea, and causes a flow down a gently-inclined plane toward the Poles. 

 In the absence, however, of more extensive and accurate knowledge of 

 the state of the ocean at great depths, or of its local direction and 

 quantity of motion, in relation to the utter stillness found generally by 

 the sounding-line to prevail in its great abysses, Sir Charles Lyell is 

 too cautious and patient a reasoner to think the time ripe for a positive 

 solution. 



