A Treatise On The Principles And Practice Of Harbour Engineering

SURVEYING, MARINE AND SUBMARINE. 57 generally of a single attendant, thougli they are sometimes in charge of two men. The life-line also acts as a communication cord, according to a code of preconcerted signals, but the most modern outfits are furnished with special speaking-tubes, or with telephonic apparatus, as also with an electric glow- light. The pump is usually double acting, worked by a couple of men, with either single, double, or triple cylinders, according to the depth of water and the pressure required. It is furnished with a gauge indicating both these data. The qualities needed in a professional diver are not exceptional, but preference will naturally be given to men of nerve and intelligence. The first descent, no doubt, is always more or less a trying experience from its very novelty. The sense of helpless confinement in the midst of a Strange and artificial environment, a feeling of oppression, and the inereased pulsation all tend to render the initial trip below water (as the writer’s experience went) somewhat uncomfortable, if not a source of trépidation. The disagreeable sensations, however, pass away with acclimatisation and practice. Almost anybody in health may make a descent in perfect safety ; but for regular and continuous work under water, full-blooded men with short necks are not desirable subjects ; neither are those suffering from palpitation or from poor and languid circulation ; nor intemperate and generally unhealthy men. Diving is said to be good for the lungs owing to the compressed air affording an inereased supply of oxygen and deepening the respiration. A diver of ordinary powers may descend to a depth of 100 feet with impunity, and may even reacli 150 feet without ill effects ; but deeper descents are not easily made, and are rarely recorded. The greatest depth to which any diver has descended by authentic testimony is 210 feet, at which point the pressure on his body was 90 Ibs. per square inch in excess of atmospheric pressure. Harbour work very seldom entails diving in water exceeding 10 fathoms in depth. The following table shows the pressures sustained over and above the ordinary atmospheric pressure at varying depths : — Depth. Feet. Pressure. Lbs. per sq. iu. Depth. Feet. Pressure. Lbs. per sq. in. 10 41 80 34| 20 84 100 434 30 12J 120 524 40 171 140 60ä 50 21j 160 69f 60 261 180 78 Care should be taken in descending and ascending not to move too rapidly—more particularly in ascending. The rate of movement should not be greater than 2 feet per second for depths less than 80 feet.