Page:Popular Science Monthly Volume 4.djvu/393

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379
MISCELLANY.

This being the case, there is little or no circulation—the water becomes to some extent stagnant, affecting in a marked manner the development of life in it.

Dr. Carpenter was astonished to find in the Mediterranean very few evidences of life at depths greater than eighteen hundred feet. "The dredge," he says, "brought up barren mud," and, however abundant life may be around its margins, its deeper portions are azoic. In the cold but freely-circulating waters of the Atlantic, animals are found at nearly the greatest depths—while the dredge is often filled from soundings of one to two miles. Whence the difference? "I found," says Dr. Carpenter, "the deep waters of the Mediterranean turbid, filled with fine particles of sediment which at last make the ooze of the bottom." The presence of this floating dust, even near the surface, is proved by the blueness of the water. Turbidity is known to be unfavorable to the development of many kinds of marine life. Prof. Dana has shown that a small quantity of sediment thrown upon a portion of a reef kills the polyps on that part, and the growth and distribution of coral-reefs are largely determined by this cause. But, another reason for the absence of life at the bottom of the Mediterranean is, the deficiency of oxygen in its waters in those depths.

Deep waters from the Atlantic and Mediterranean have been boiled off, and in the gases from the first there was twenty per cent, of oxygen, from the latter only five percent. But of carbonic acid there was from the first only thirty to forty per cent., while the latter furnished sixty per cent. Here, then, in the abundance of carbonic acid, and deficiency of oxygen, is a possible cause for the paucity of life.

But whence arises the deficiency of oxygen. Chiefly, perhaps, from the slow decomposition of organic matters, carried in by rivers and other agencies, and which may add to the turbidity referred to.

This state of things in the Mediterranean, and possibly in other inland seas, evidently arises from absence of circulation of the waters. Winds disturb the surface only, and Dr. Carpenter says there is in the Mediterranean no thermal circulation—or circulation which arises from inequality of temperature. It has been said that inequality of density caused by evaporation must produce some vertical circulation of the water, but density from lowering of temperature at the surface never exceeds the density of the deeper waters, and no circulation disturbs or mixes the superimposed masses.

The oxygen from the surface can only reach the deeper waters by diffusion, hence its deficiency at considerable depths. In the Atlantic, and doubtless in all open oceans, the waters are diffused, and mixed by a wonderful system of circulation, the dynamic agencies of which are not present, or only in a modified form, in the inland basins.

The well-known and often-criticised statement of Edward Forbes, that life ceases at a depth of three hundred fathoms, is confirmed by the researches of Dr. Carpenter, in so far as it relates to inland seas, the researches of Prof. Forbes being in the Ægean. The error consisted in applying the same rule to the open oceans, where a different one prevails, and this appears to have been the error of others rather than of Forbes.

 

Unequal Power of the Eyes.—Probably there are but few persons possessed of equal power of vision in both eyes. This circumstance, as is observed by a writer in Science Gossip, will doubtless account for some people being unable to appreciate the binocular microscope. The writer in Science Gossip has a friend who always found difficulty in studying with a binocular, in that he could never get the two glasses to blend. In 1851 he attended the Great Exhibition in London, and there his eyes were constantly ranging from short to long distances. After he had left the Crystal Palace he felt that his eyes were very much fatigued, and was at a loss to understand the meaning of it. By this and other circumstances he discovered that there was a focal difference in his eyes. One eye was far-sighted, while the other was near-sighted. For reading-purposes he wears a pair of spectacles in which the one glass is made for the far sight, while the other is a plain glass, the left eye being near-sighted, and consequently requiring no aid from specta-