Page:Popular Science Monthly Volume 1.djvu/249

Rh 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 results 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, however, is felt by Sir Charles to support his previously-expressed conviction 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 Mediterranean and the Atlantic, as well as the difference of four degrees between the deep-sea soundings of the western and central basins of the Mediterranean 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 circulation arising out of the extreme cold found at great depths both in temperate 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 displacement, 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.