218 THE POPULAR SCIENCE MONTHLY. 



throw upon the water to amuse ourselves with their vain strug- 

 gles to get under it. 



Then conies 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 con- 

 sists 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 ossi- 

 cles, 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 feat- 

 ures 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 de- 

 sires 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 unex- 



