644 EVENING DISCOURSES. 



pauses. This can be effected by having a series of air-locks, and making the men 

 walk, or better, climb between each. In the East River tunnels this method was 

 tried' with good results— (1) +40 to +29 lb. in 5 minutes; (2) 10 minutes walk- 

 ing in +29 lb.; (3) +29 to + 12£ lb. in 8 minutes; (4) 10 minutes walking in 

 +12£ lb.; (5) +12J* to +0 in 15 minutes. Lengths of tunnel were arranged 

 between locks for walking in. Total time, 48 minutes. The Admiralty table 

 enforces 97 minutes for this pressure. 



As there were 160 per cent, cases of ' bends ' and no serious ones— the 

 Admiralty time is demonstrated to be unnecessarily long. This is particularly 

 so if hard work is done during decompression, for the same amount of nitrogen 

 would be expelled in about one fifth of the time as during rest. 



Greenwood and I have tested the stage method on pigs which are more like 

 men in shape, diet, and habit than goats — the animals used in the investi- 

 gations conducted for the Admiralty Committee. It appears from our results 

 fairly safe to decompress even fat pigs from 6 atmospheres to 2£ atmospheres in 

 about ten minutes, and then after a pause of 1£ hour from 2| to 1 atmosphere 

 in twenty minutes. The pigs slept quietly in the warm caisson, and never 

 moved, and being fat, were very unfavourable subjects. One death and no severe 

 case of illness occurred among 47 pigs weighing 50 to 100 lb. ; one severe and 

 three slight cases among 19 goats weighing 39 to 57 lb. A similar decompression 

 of fat pigs from 7 atmospheres, allowing 105 to 120 minutes interval at 2£ atmo- 

 spheres, gave unfavourable results, seven deaths and one severe case — among 27 

 pigs weighing 81 to 115 lb. Only one pig out of all showed any symptoms after 

 reaching the stage at 2£ atmospheres. At these very high pressures there is great 

 risk unless time enough is given, and plenty of exercise taken during the pause. 

 For pressures up to 4 atmospheres the method employed by Mr. Yapp at the 

 East River Tunnel is evidently a very good one. For pressure 2 to 3 atmospheres 

 it is an advantage to do work immediately after decompression, supposing 

 work cannot be provided between two air-locks (Bornstein). At the Greenwich 

 tunnel, now being built, the men climb the shaft 60 feet high after decom- 

 pression, and since I made the suggestion, and the engineer, Mr. E. H. Tabor, 

 carried it out, the number of cases of 'bends' has dropped from 1 in 94 

 to 1 in 240 man-shifts. For higher pressures it would not be safe to take exer- 

 cise after, it ought to be taken during decompression and the pauses between the 

 stage decompressions. The importance of this cannot be insisted on too much. 

 Exercise during decompression is the simplest means of rendering comprassed-air 

 work safe, and of keeping the period of decompression of a reasonable length. 



The question of the length of shift desirable has been much discussed. Long 

 shifts of 8 hours are found to give more illness than shifts of, say, 1 to 2 hours. 

 Every practical caisson engineer agrees to that. Divers are decompressed in a few 

 minutes from high pressures (5 to 6 atmospheres), with comparative immunity, 

 if they have been down for only a few minutes. Cases of illness occur when they 

 exceed their stay, or after a succession of dives, each of which helps to 

 saturate slow parts and increases the fatigue of the diver. The Admiralty 

 table fixes the period spent at the bottom so as to prevent saturation of ' slow ' 

 parts and shorten the period of decompression. The descent is hastened for the 

 same reason. It is quite safe to descend to 200 feet in 2 minutes ; slow descents 

 only increase risk by increasing the saturation of the body. In the matter of 

 the caisson- workers, at the East River Tunnel, two 3-hour shifts per diem, 

 with 3 hours' rest between, gave 1'07 per cent, cases, and one 8-hour shift 062 

 per cent, cases. The men are so far saturated in 3 hours of hard work, 

 that doubling the decompressions is worse than extending the shift to 8 hours. 

 As bubbles may persist for a long time in the tissues, and may act as starting- 

 points for the formation of other bubbles, it is wise to give long intervals of 

 time between shifts — also in a short interval slow parts may not become de- 

 saturated. Haldane has suggested the men should return to a 'purgatory' 

 chamber — say, at 2 atmospheres, and eat their dinner and rest there in the mid- 

 period of an 8-hour shift and again at the end of the shift when, while waiting 

 for decompression to 1 atmosphere, they could wash, change their clothes, and have 

 some hot coffee to stimulate the circulation. In any large tunnel-works such a 

 chamber could be easily constructed out of a section of the tunnel. This would 

 suffice for stage decompression, and would give excellent results if the men 

 could be persuaded to take exercise in it, or be given oxygen to breathe before 

 decompression to 1 atmosphere. 



