Fig. 161.—A diver's helmet.
By means of telephone wires built into the life-line (which passes under the diver's arms and is used for lowering and hoisting) easy communication is established between the diver and his attendants above. The transmitter of the telephone is placed inside the helmet between the front and a side window, the receiver and the button of an electric bell in the crown. This last he can press by raising his head. The life-line sometimes also includes the wires for an electric lamp (Fig. 162) used by the diver at depths to which daylight cannot penetrate.
The pressure on a diver's body increases in the ratio of 4⅓ lbs. per square inch for every 10 feet that he descends. The ordinary working limit is about 150 feet, though "old hands" are able to stand greater pressures. The record is held by one James Hooper, who, when removing the cargo of the Cape Horn sunk off the South American coast, made seven descents of 201 feet, one of which lasted for forty-two minutes.
Fig. 162.—Diver's electric lamp.
A sketch is given (Fig. 163) of divers working below water with pneumatic tools, fed from above with high-pressure air. Owing to his buoyancy a diver has little depressing or pushing power, and he cannot bore a hole in a post with an auger unless he is able to rest his back against some firm object, or is roped to the post. Pneumatic chipping tools merely require holding to their work, their weight offering sufficient resistance to the very rapid blows which they make.
Fig. 163.—Divers at work below water with pneumatic tools.