299 The practice of diving obliges the diver to conduct his work under a pressure greater than that of the atmosphere at the surface of the earth. All diving work is done under an abnormal atmospheric pressure, which increases with the depth at which the diver is submerged in water. This pressure, when he is submerged to the depth of 33 feet, is twice that of the normal superficial atmospheric pressure. At greater depths the pressure is proportionately increased, and ultimately becomes so great that life could not be maintained. To descend even to the moderate depth of 30 or 40 feet, which is about the maximum required for ordinary engineering sea works, demands some practice and nerve on the part of the diver, but when greater depths have to be explored, in raising sunk vessels, for example, the energy and power of endurance of the diver are much more severely taxed, and it seems not uninterest ing, before concluding this article, to refer to the effect which the work has on the health of the diver, as well as on some physiological facts of interest in general science, nsations The sensations experienced in a diving bell ars common, peri- it is believed, to all divers. According to the writer s ex- ^ perience, very soon after the lips of the bell have touched the surface of the water pain is felt in the ears and above the eyes, which continues with greater or less intensity according to the rate of descent until the bell has attained the bottom. So long as the bell continues there no pain is felt, the only feeling being that of depression due to the depth to which the diver is submerged. As soon as the upward movement commences the pain in the ears and above the eyes returns, and continues till the surface is reached. The motion of the bell is very gradual, sometimes not exceeding 3 feet per minute, but even at that slow rate the head does not accommodate itself to the increase of pressure so as to avoid inconvenience. Aeronauts do not suffer to the same extent in their ascents in balloons, because the alteration of pressure is much more gradual in passing through the atmosphere than through a medium having the density of water. Several suggestions have been offered as accounting for the sensations which are experienced in diving, and the following explanation, which the author has submitted to Professor Turner of Edinburgh, is believed to afford the true solution. Under the ordinary atmospheric conditions, the air presses not only on the surface of the body, but into every cavity within the body which communicates with the sur face, so that the pressure, both externally and internally, is exactly balanced. In passing into a denser atmosphere the increased pressure operates externally more rapidly than it does internally, more especially if the communica tion of the internal cavities with the surface is by tortuous passages ; and so long as this inequality in the pressure exists the disagreeable sensations in the ears and above the eyes will continue. The pain in the ears arises from the effect of the condensed air acting externally on the tympanic membrane of the ear, before the air within the tympanic cavity has acquired the same density to counter balance it. The tympanic membrane stretches across the bottom of the passage or meatus, which leads from the outer ear into the side of the head (see ANATOMY, fig. 80.) This passage is in direct communication with the atmo sphere, the pressure of which, therefore, acts instantaneously on the tympanic membrane. But on its inside the tympanic membrane bounds the tympanic cavity, which has no communcation with the external air, excepting by the Eustachian tube, which leads from the cavity into the pharynx immediately behind the nose. Through this tube, therefore, the condensed air must pass from the pharynx to supply what is necessary within the cavity for restoring tiie same equilibrium within and without. But the Eustachian tube is a long and narrow passage ; at its com mencement in the ear it has a bony structure, but towards its termination in the pharynx behind the nostrils, it becomes soft, so that its walls can be forced together. Tt admits an easy passage from the ear to the pharynx ; but when any pressure arises in the opposite direc tion, it acts in some degree like a valve, shutting the passage, until the increasing pressure again forces it open. Some time then elapses before all this can be accomplished ; and during this time the external air, pressing with full force on the tympanic membrane, produces the pain which is felt. When the Eustachian tube opens, it is generally all of a sudden, and with a slight explosion or pop, which is followed by instant relief from the pain. This relief may often be produced by filling the mouth, or gulping the air and passing it into the tube. That the above is what really takes place may be shown experimentally by shutting the mouth and nostrils, and exhausting the air from them by the action of the lungs. The air in the tympanic cavity immediately rushing through the Eustachian tube into the mouth, the external air acts on the tympanic membrane and produces a slight sensation of deafness, such as is felt in the bell. But if, instead of exhausting the air, we attempt to compress it, and force it through the tube into the tympanic cavity, at first no effect is produced ; but after exerting a considerable pressure a slight pop is felt, and a little pain in the ear, which is just the sudden opening of the tube. The pain above the eyes is doubtless due to the inequality between the pressure of the air on the surface of the fore head and that of the air in the frontal sinuses, or air spaces in the frontal and other bones which form the boundaries of the orbits. The return of the disagreeable sensations during the upward ascent of the bell is due to the pressure on the outer surface of the tympanic membrane and of the forehead being diminished, before the air within the tympanic cavity and the air spaces in the bones of the orbits has accommodated itself to the diminished external pressure. It may further be interesting to notice that any upward motion is accompanied by a thick mist within the bell, which disappears when it is stationary or moving down~ tcards. The explanation is that the air inside the bell, when it is ascending, being relieved of pressure, expands, and its temperature is lowered ; and as the air inside is about the point of saturation, the fall of temperature pro-- duces condensation, which becom-es visible in the form of vapour or mist. An analogous phenomenon takes place in commencing to exhaust the receiver of an air-pump*. The question of the effect produced on the health of the Effect men employed in diving is of interest and importance, diving So far as the author s experience goes, he is not aware that e . divers suffer from prosecuting their submarine work under ( j ivers the pressure of one or two atmospheres to which they aro subjected in ordinary harbour works, the men selected for such duty being generally healthy young men of athletic make. Indeed, it is well known that to some constitutions, and in some forms of disease, subjection to moderate increase of atmospheric pressure proves beneficial. But when greater depths and high pressures have to be sustained the case may be very different. Mr Siebe, who states the greatest depth to which a diver has descended to be 201 feet, with a pressure of 87 Ib on the square inch (but who states 150 feet as the limit for safe work), has given various directions, the result of hi3 experience, as to the selection of men for deep diving, and advises that men should not be employed who are of full habit of body, who suffer from headache or deafness, who have at any time, had spitting of blood or palpitation of
the heart, who are pale and whose circulation is languid,