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

Absolute Pressure. Air Pressure in the Helmet. Helmet Valves and their action. above atmospheric pressure. These facts constitute what is called “ Boyle’s Law,” which is that the volume of a gas varies inversely as the pressure, whilst the density varies directly as the pressure. It is not necessary lor us to complicate this simple rule by dealing with the exact effect of changes in temperature of the air; but it is well to remember that if the temperature of confined air is raised the pressure will be increased, and if the temperature is lowered the pressure will be decreased. When dealing with water totally different conditions arise ; since water can- not be compressed, it is, of course, a practical impossibility to lorce a quart ol water into a pint measure. Another important consideration is that water is very much heavier than air, and that, although when dealing with ordinary quantities of air we may neglect its weight, we cannot do so where water is concerned. If a pint measure be full of air we can neglect the weight of the pint of air on the bottom of the measure, but it it be filled with water we cannot neglect the weight of water on the bottom. It is obvious that if a cubic box be filled with water the bottom of the box has to support the whole weight ol the water, but the top has no weight of water to support ; the pressure therefore on the bottom will be greater than the pressure on the top by an amount equal to the weight of water in the box. It is also obvious that the higher we raise the sides of the box, still keeping it filled up, the greater the weight or pressure becomes on the bottom. A column of water 33 feet in height presses on the bottom of the column with a pressure of about 15 lbs. on the square inch. This is the same pressure as is produced by one atmosphere ol air, or a column of air the height of the atmo- sphere. Sixty-six feet of water would produce at its base a pressure ol about 30 lb., or the same as two atmospheres, and so on. in other words, every one toot in height of salt water produces a pressure of a little under lb. on the square inch. It must be remembered that the atmosphere is also pressing down on top of the column of water, so that a column of water 66 feet in height would have at its base an absolute pressure of 30 lbs. due to its weight, plus 15 lbs. due to the atmosphere, or a total of 45 lbs. absolute pressure. The same holds good when considering the pressure on a body immersed in water. Any such body may be looked on as having the column of water between it and the surface pressing down all round it. 1 his pressure is transmitted to the body in the form of a squeeze. Of course, it the body has appreciable length, such as a diver standing upright, the top of the body is nearer to the surface ol the water than is the bottom, therefore there is less water above the top ol the body than above its bottom, and therefore in the case of the diver there is less pressure on his helmet than on his boots. 11 the diver be 6 feet high there will be about 3 lbs. less pressure on his helmet than on his boots, whatever depth of water he may happen to be in. We are now in a position to see the connection between the pressure of the air inside a diving dress and of the water outside it. A diving dress has essen- tially two portions, a compressible dress and an incompressible helmet. As the diver descends the pressure increases, and tries to squeeze the air out ot the drc ss into the helmet. If we wish to keep the upper part of the dress over the man’s chest inflated, air must be pumped into the helmet and the upper part ol the 10