Greatest Depths attained.—The greatest depth at which useful work has been performed by a diver is 182 ft. From this depth a Spanish diver, Angel Erostarbe, recovered £9000 in silver bars from the wreck of the steamer “Skyro,” sunk off Cape Finisterre; Alexander Lambert succeeded in salving £70,000 from the Spanish mail steamer “Alphonso XII,” sunk in 162 ft. of water off Las Palmas, Grand Canary; W. Ridyard recovered £50,000 in silver dollars from the “Hamilton Mitchell,” sunk off Leuconna Reef, China, in 150 ft. There are individual cases where much larger sums have been recovered, but those mentioned are particularly notable by reason of the great depth involved and stand out as the greatest depths at which good work has been done. The sponge fishers of the Mediterranean work at a maximum depth of about 150 ft., and the pearl divers of Australia at 120 ft. But submarine operations on the great majority of the harbour and dock works of the world are conducted at a depth of from 30 to 60 ft.
The weighted tools employed by divers differ very little from those used by the workmen on terra firma. Pneumatic tools, worked by compressed air conveyed from the surface through flexible tubes, are great aids, particularly in rock removal work. With the rock drill the diver bores a number of holes to a given depth, inserts in these the charges of dynamite or other explosive used, attaches one end of a wire to a detonator which is inserted in the charge, and then comes to the surface. The boat from which he works is then moved away from the scene of operations, paying out the wire attached to the detonators, and when at a safe distance the free end of the wire is connected to a magneto exploding machine, which is then set in motion.
A complete set of diving apparatus costs from £75 to £200, varying with the depth of water for which it is required.
The pay of a diver depends upon the nature of the work upon which he is engaged, and also upon the depth of the water. On harbour and dock work the average wage is 2s. to 2s. 6d. per hour; on wreck work from 3s. to 5s. an hour, according to depth; on treasure and cargo recovery so much per day, with a percentage on the value recovered, generally about 5%. The pearl fishers of Australia get so much per ton of shell, and the sponge fishers are also paid by results.
A problem which has been exercising the minds of those engaged in submarine work is the greatest depth at which it is possible to work, for, as is well known, many a fine vessel with valuable cargo and treasure is lying out of reach of the diver owing to the pressure which he would have to sustain were he to attempt to reach her. Mr Leonard Hill, and Drs Greenwood and J. J. R. Macleod conducted experiments in conjunction with Messrs Siebe, Gorman & Co., with a view to solving this problem, and their efforts have been attended with some considerable success. Dr J. S. Haldane has also carried out practical experiments for the British Admiralty, and under his supervision two naval officers have succeeded in reaching the unprecedented depth of 210 ft., at which depth the pressure is about 90 ℔ to the square inch.
Diving Bells.—Every one is familiar with the experiment of placing an inverted tumbler in a bowl of water, and seeing the water excluded from the tumbler by the air inside it. Perhaps it was to some such experiment as this that the conception of the diving bell was due. As is well known, the pressure of water increases with the depth, and for all practical purposes this pressure can be taken at 4¼ ℔ to every 10 ft. The following table shows the pressure at different depths below the surface of the water:—
| Depth. | Pressure. |
| 20 ft. | 8½ lb to the sq. in. |
| 40 ” | 17¼ ” ” |
| 80 ” | 34¾ ” ” |
| 120 ” | 52½ ” ” |
| 160 ” | 69¾ ” ” |
| 200 ” | 87 ” ” |
If a diving bell be sunk to a depth of, say, 33 ft., the air inside it will be compressed to about half its original volume, and the bell itself will be about half filled with water. But if a supply of air be maintained at a pressure equal to the depth of water at which the bell is submerged, not only will the water be kept down to the cutting edge, but the bell will be ventilated and it will be possible for its occupants to work for hours at a stretch.
Tradition gives Roger Bacon, in 1250, the credit for being the originator of the diving bell, but actual records are lost in antiquity. Of the records preserved to us, probably one of the most trustworthy is an account given in Kaspar Schott’s work, Technica curiosa, published in the year 1664, which quoted from one John Taisnier, who was in the service of Charles V. This account describes an experiment which took place at Toledo, Spain, in the year 1538, before the emperor and some thousands of spectators, when two Greeks descended into the water in a large “kettle,” suspended by ropes, with its mouth downwards. The “kettle” was equipoised by lead fixed round its mouth. The men came up dry, and a lighted candle, which they had taken down with them, was still burning.