Submarine Appliances And Their Uses
Deep Sea Diving, &c., &c.

THE PHYSICS AND PHYSIOLOGY OF DIVING.* THE PHYSICS OF DIVING. It is necessary before discussing the effects of deep diving on the human body to review very briefly a few of the physical properties of air and water. Atmospheric If a pint measure be standing empty in the air, every square inch of its Pressure. surface, inside as well as outside, is being pressed on by the air with a pressure of, roughly, 15 lbs. This is the amount of pressure that the atmosphere all round us approxi- mately maintains on everything near the surface of the earth. It is due to the weight of the miles of air overhead pressing down towards the surface of the earth. This pressure of 15 lbs. on the square inch that the atmosphere exerts is called “atmospheric pressure,’’ or the “ pressure of one atmosphere.” Such pressure is not usually shown by a gauge, since gauges generally register a pressure in excess of that which is always present due to the atmosphere. There arc, therefore, two ways in which pressure can be reckoned—first, that which includes the pressure exercised by the atmosphere, this is called “ absolute pres- sure ” ; and secondly, that which docs not take the pressure of the atmosphere into account; the latter is that generally shown by gauges, and is usually talked of as “ pressure. ” If now the pint measure be closed at the top, and another pint of air be forced into it, there will be a double quantity of air in the measure trying to force its way out in all directions ; the original pressure will therefore be doubled, and there will be an absolute pressure of 30 lbs. on the square inch inside the measure on the top, bottom, and sides; but, of course, only 15 lbs. “ pressure” above that of the atmosphere inside the measure. The same result could have been obtained if an india-rubber ball containing a quart of air had been pressed on from the outside, and the ball squeezed in until its capacity had been reduced to one pint. A quart of air (that is, two pints of air) would then be held in one pint of space, and the pressure inside it therefore doubled. It is convenient to use the word “volume ” to express the quantity of air in any receptacle at atmospheric pressure. The following simple rule can, therefore, be used, viz. : If one volume of air is present in a receptacle, the pressure is atmospheric pressure, or 15 lbs. absolute pressure. If two volumes are forced in, the pressure is doubled. If three volumes, the pressure is trebled, and so on. Further, if it is desired to obtain any given pressure of air in a receptacle, it is only necessary to force in a certain number of volumes of air to obtain it. For in- stance, if a pressure of 90 lbs. on the square inch above atmospheric pressure is required, we have merely to divide 90 by 15 to see how many volumes of air, in addition to the one already present due to the atmosphere, must be forced in. In this case six more volumes will produce a pressure of 90 lbs. on the square inch * Reproduced by permission of the Admiralty. 9