To avoid this risk, it is necessary for the diver working at great depths to come up very slowly. He may slide down the shot-rope to a depth of 120 feet in a few seconds, but, should he stay longer than half an hour at the bottom, he must not come up in less than fifty-seven minutes if he would avert danger. He may come up to 40 feet in eighty seconds, or at the rate of a foot a second. Then he must rest and exercise his legs and arms on the shot-rope for five minutes before ascending another 10 feet to the 30-foot level. Here he must rest for a further period of fifteen minutes, and do those exercises which help to rid his muscles of their excess of nitrogen. Ascending another 10 feet, which brings him to within 10 feet of the surface, he is compelled to rest for twenty-five minutes to allow the excess of nitrogen to pass from his blood, after which he may rise to the surface. If a diver happened to remain an hour at a depth of 200 feet, he would have to spend four hours in coming to the surface to avoid any ill effects. The exceptional diver who is able to reach this depth should not, however, remain at the bottom for more than twelve minutes. This is the safe time, and he can then make the ascent to the surface in thirty-two minutes. Remarkable diving experiments were carried out by the British Admiralty some years ago, during which naval divers attained the record depth of 210 feet, a record that was long unbeaten. As a result of these experiments, tables were drawn up showing the time that a man might remain in safety at certain depths, and indicating the rates at which he could come to the surface and the depths at which he must rest to allow the pressure inside his body to adjust itself to the pressure of the water outside. These tables are followed the wide world over, and they have made diving one of the safest of occupations, despite the grave risks the diver is continually running. Diving was, in fact, so dangerous that exceptional precautions had to be taken, with the result that the diver who walks about the bottom of the ocean to-day may be far safer than a man walking across Piccadilly Circus. The safety of the diver is most carefully watched over, but no one can foretell when a motor vehicle is going to run down some one crossing a busy road.