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

Pressure effects. dress until the pressure of air in the dress is equal to that of the water at the level of his chest. As the man breathes, the air gradually becomes foul ; fresh air must therefore be pumped in for him to breathe. If no escape is provided for the air from the dress, the pressure inside the dress would get higher and higher, and there would be more pressure inside the dress than outside it, which for several reasons would be very objectionable. An escape valve for this surplus air is therefore fitted. This escape valve works so that when the pressure inside the valve is greater than that of the water outside the valve, it is forced open and the extra air escapes. At first it seems as if the proper place for this valve would be at the same level as the man's chest, since it is the pressure at that level which is required in the dress'; but many practical considerations lead to placing the valve at the back of the helmet. Since, when the man is standing upright, this spot has a pressure about | lb. less than exists at the man’s chest, a spring is fitted to keep the valve shut until the pressure inside has risen about | lb. more than that of the water outside ; by this means air is kept in the dress over the man’s chest, although the valve itself is a little nearer the surface of the water. This valve can be adjusted by the diver to suit his convenience. This is very ne- cessary, since a diver often lies down, when the relative depth of his chest and the valve is altered. Also, it makes a very great difference to the diver if the exactly right inflation of his dress is obtained. If the dress is too much inflated he be- comes uncomfortably buoyant, and if too little inflated the weights bear hcavilv on his chest. It is, moreover, much less exertion when breathing inside a small space to have an clastic bag attached to it, like the inflated part of the dress, than to "breathe inside such a space with rigid walls like the helmet alone, since the elastic part u gives ” to each breath and keeps the pressure constant. The lungs also are only designed to work comfortably when the pressure inside and outside them is exactly the same ; very little additional pressure outside them greatly increases the labour of breathing. As regards the remainder of the body, the pressure of the air at the mouth is instantly transmitted to the whole inside of the body. So that whatever depth the diver may be in, provided a proper air supply is maintained, the pressure outside the body can never be more than a pound or two over that inside the body. It must be remembered that air cannot be supplied to the inside of the dress by the pumps at more than a certain rate, hence, if for any reason a diver changes his depth at a greater rate than that at which the pumps can supply him with the proper pressure, he may be subjected to a very dangerous squeeze from the water. Falls under water. The following illustration will emphasise this :— Suppose a diver at work on a stage cleaning the ship’s bottom slips off the stage und falls 5 fathoms, owing’ to carelessness on his psrt, und culpable neglect on the part of his attendants to hold on to the life line and air pipe. At this depth the absolute pressure is twice as much as it is close to the sui face, so by the descent the volume of the air in his dress is halved. He has an additional pressure of nearly 15 lbs. suddenly applied to every square inch of his body, or about 2,000 lbs. to every square foot. As the helmet is rigid, his body will be crushed into it with overwhelming force. He may be killed, he certainly will be severely injured, and there will be bleeding from the lungs, mouth, nose, etc. If, however, the diver is at 165 feet (27J fathoms) and falls 5 fathoms, pressure has only increased in the proportion of seven to six, and the volume of air has only diminished by less than one-seventh of its volume, so that the effect will be proportionately less. From this it follows that the deeper a diver is the less serious is a fall. 11