On the surface are the air-pumps, which may be of several types—single-cylinder, double-acting; double-cylinder, double-acting; or three or four cylinder, single-acting—according to the nature of the work. All patterns are so constructed that the valves may be easily removed and examined.
The pressure on a diver increases in the ratio of about 4 1 / 4 lbs. for every ten feet he descends below the surface. A novice experiences severe pains in the ears and eyes at a few fathoms' depth, which, however, pass off when the pressures both inside and outside of the various organs have become equalised. On rising to the surface again the pains recur, since the external pressure on the body falls more quickly than the internal. The rule for all divers, therefore, is "slow down, slow up." Men of good constitution and resourcefulness are needed for the profession of diving. Only a few can work at extreme depths, though an old hand is able to remain for several hours at a time in sixty feet of water. The record depth reached by a diver is claimed by James Hooper, who, when removing the cargo of the Cape Horn, wrecked off the coast of South America, made seven descents to 201 feet, one of which lasted forty-two minutes.
In spite of the dangers and inconveniences attached to his calling, the diver finds in it compensations, and even fascinations, which outweigh its disadvantages. The pay is good—£1 to £2 a day—and in deep-sea salvage he often gets a substantial percentage of all the treasure recovered, the percentage rising as the depth increases. Thus the diver Alexander Lambert, who performed some plucky feats during the driving of the Severn Tunnel,[18] received £4,000 for the recovery of £70,000 worth of gold from the Alphonso XII., sunk off Grand Canary. Divers Ridyard and Penk recovered £50,000 from the Hamilla Mitchell, which lay in 160 feet of water off Shanghai, after nearly being captured by Chinese pirates; and we could add many other instances in which treasure has been rescued from the maw of the sea.
The most useful sphere for a diver is undoubtedly connected with the harbour work and the cleaning of ships' bottoms. For the latter purpose every large warship in the British Navy carries at least one diver. After ships have been long in the water barnacles and marine growths accumulate on the below-water plates in such quantities as to seriously diminish the ship's speed, which means a great waste of fuel, and would entail a loss of efficiency in case of war breaking out. Armed with the proper tools, a gang of divers will soon clean the "foul bottom," at a much smaller cost of time and money than would be incurred by dry-docking the vessel.
The Navy has at Portsmouth, Sheerness, and Devonport schools where diving is taught to picked men, the depth in which they work being gradually increased to 120 feet. Messrs. Siebe and Gorman employ hundreds of divers in all parts of the world, on all kinds of submarine work, and they are able to boast that never has a defect in their apparatus been responsible for a single death. This is due both to the very careful tests to which every article is subjected before it leaves their works, and also to the thorough training given to their employés.
In the sponge and pearl-fishing industries the diving dress is gradually ousting the unaided powers of the naked diver. One man equipped with a standard dress can do the work of twenty natural divers, and do it more efficiently, as he can pick and choose his material.
This chapter may conclude with a reference to the apparatus now used in exploring or rescue work in mines, where deadly fumes have overcome the miners. It consists of an air-tight mask connected by tubes to a chamber full of oxygen and to a bag containing materials which absorb the carbonic acid of exhaled air. The wearer uses the same air over and over again, and is able to remain independent of the outer atmosphere for more than an hour. The apparatus is also useful for firemen when they have to pass through thick smoke.
FOOTNOTE:
[18.] Vide The Romance of Modern Engineering, p. 212.