The circulation of the blood and the breathing of whales (including in that term the smaller kinds known as dolphins and porpoises) is still a matter which is not properly understood. When a Greenland whale is struck by the harpoon it dives vertically downward to a depth of 400 fathoms and more (nearly half a mile), and occasionally wounds the skin and bones of its snout by violently striking it on the sea-bottom. It remains below as long as forty minutes. Physiologists wish to know how the sudden compression of the air in the lungs in plunging to this depth and the equally sudden expansion of it in rising from such a depth is dealt with in the whale's economy, so as to prevent the absolutely deadly results which would ensue were any ordinary air-breathing animal subjected to such changes of pressure. Man can endure without suffering an increase of pressure of the gases in his body amounting to three or four times that to which he is accustomed, as, for instance, when working in the compressed air of "caissons." But the whale goes suddenly to a depth at which the pressure is eighty times that at the surface! Then, too, man (and other terrestrial animals), after being subjected (for instance, in a caisson) to a pressure of four times that which exists on the free surface of the earth, is liable to be killed by suddenly passing from that high pressure into the ordinary air. The gases dissolved in his blood expand like the gas in a bottle of soda-water when the cork is drawn, and the bubbles interfere with the circulation of the blood in the finer blood-vessels (of especial importance being those of the brain and spinal cord), and the serious illness and the death of workmen has frequently resulted from this cause. Accordingly, the men who work in such "compressed atmospheres" are now made to pass slowly through a series of three chambers, in each of which the pressure is diminished and brought nearer to that of the normal atmosphere. By spending twenty minutes in each chamber successively, the workman is gradually brought to the pressure of the outer world, and his blood prevented from "effervescing." But what must be the condition of the gases in the blood of a whale which suddenly rises from 400 fathoms to the surface? The whale suddenly goes, not from a pressure of four times the normal ("four atmospheres," as it is called), but from eighty times the normal, to the normal pressure.