Page:Popular Science Monthly Volume 38.djvu/230

218 throw upon the water to amuse ourselves with their vain struggles to get under it.

Then comes the gurgling of the water and air escaping through the valve, and you descend. The pressure immediately increases at the rate of one atmosphere for about every ten metres of depth. This increased pressure, which would he insupportable if it was unequally distributed, is hardly felt, because it is exerted in every direction. The air is reduced to half its former volume, so that our inspirations take in double the usual quantity. Instead of breathing more easily, as one would naturally suppose he would do, the diver feels an oppression which is very troublesome at first. But it soon passes away. It is caused by a pressure on the alveoli of the lungs which impedes the exchange of gases. But the equilibrium is soon restored spontaneously.

The most disagreeable sensation produced by the descent consists of pains in the ears, sharp and accompanied by a feeling of dizziness. It is caused by a pressure of the air contained in the medial ear; the tympanum is stretched and pushed upon the ossicles, till a bubble succeeds in making a passage for itself through the Eustachian tube. The pain then ceases, but returns as the descent is continued. After a few plunges, the Eustachian tube enlarges enough to let the air pass freely, and the pains cease. The dizziness is explained by the fact that the inner ear, as M. Delage and other physiologists have shown, is the seat of the sense of direction; so the novice does not know where he is, and imagines that his head is down. Mariners, in training for diving, are caused to go down first in a spot where there is hardly water enough to cover the casque; they come back with downcast features and the flurried air of a man afflicted with vertigo.

The most delicate point is the regulation of the air-escape. The novice lets out too much air, and water comes in by the valve, and the casque seems so heavy that he imagines he is nailed to the bottom. He then lets too much air accumulate, his coat swells, and the casque rises so much as to take the valve out of reach of the hand. Despite all his efforts to stay on the bottom, he springs up to the surface. The air, released from the pressure, expands, the coat is inflated almost to bursting, and he floats like a dead body. One can never be a good diver till he learns to regulate the air as a horseman holds the reins—without thinking about it. We might, indeed, adjust the valve for a particular depth, so that it shall act automatically; but the diver who desires to ascend and descend at will, will do better to keep the escape-valve taut, and regulate it with his head.

The beginner is not able to travel about as he wants to—first, because he feels too light or too heavy, according to the quantity of air in his coat; and, second, because the water offers an