Coiseau suffered when they had been making too free with the whisky overnight, and a good dual of the disorders that ensued were traceable to the same source; though, on the other hand, wet feet, or incautious and sudden change from a heated atmosphere into a cold and biting east wind, insufficiency of clothing, and want of proper nourishment, had their influence in causing illness among the workers. Although these six caissons were founded at depths varying from 63 ft. to 89 ft. below high water, with an average time of seventy-eight days for each, not one death can be properly and justly attributed to working in high pressure. Two men died during the time, but both were already consumptive when they commenced working here, and the rigour of a Scotch winter had, at any rate, as much to do with their death as the air pressure. Another man became insane, and had to be sent back to his own country.
The principal bad effect produced by the air pressure appears to be that of severe pains in the joint8 and muscles of the arms and legs. As these have been, in most cases, traced to hard work and consequent copious perspiration, and also to too long a stay under pressure, it has been suggested as a probable cause that small globules of air make their way through the skin, or between the skins, where they remain, and on the workman returning to ordinary atmospheric pressure, expand, and thereby cause the most agonising pains in the joints, the elbows, shoulders, knee-caps, and other places. In seeming confirmation of this, the sufferers got instant relief on returning into the high pressure. Thus it happened that many of those afflicted with this disorder spent the greater part of Saturday afternoon and Sunday under air pressure, and only came out when absolutely obliged to do so. Various researches wore made by members of the medical staff in the endeavour to give relief or obtain a cure, but, so far, not with any degree of success.
Sinking of the Caissons.
The operation of sinking a caisson through mud, silt, or clay differs somewhat from that pursued in sinking through solid rock, at any rate in the mode of attack.
In the Queensferry caissons the first thing to be done was to fix pipes in the air-shaft, one for the admission of water, the other for the removal of mud diluted with water. As this process of ejecting matter will be described further on, it need only be mentioned that by degrees the shaft first, and next a space underneath it in the air-chamber, was cleared, and access had to the latter. As soon as the ejector pipes described above could be reached, flexible hose was attached to them, and a larger number of men employed to clear the caisson of the semi-fluid mud. Great care was required during this early stage of the work, and the weight of the caisson regulated to a nicety, for there was nothing but its buoyancy to prevent it from suddenly descending and smothering the men below. At low water was naturally the most dangerous time, there being the least displacement in action then, while the weight was greatest, and moreover the cutting edge resting upon a treacherous ground. The men were generally withdrawn at this time, and the air pressure diminished, allowing the caisson to descend as far as it would go. On one of these occasions the caisson suddenly descended some 7 ft., and not only the air-chamber but part of the ascending shaft became filled again with mud and silt. It is satisfactory to be able to say that not a single life was lost here from this cause, which in other works had proved fatal to so many.
_Fig._47,_Page_27.png)
Fig. 47. Section of Tilted Caisson after Completion.
_Figs._49_and_50,_Page_27.png)
HYDRAULIC SPADES
_Fig._48,_Page_27.png)
Fig. 48. Sinking the Queensferry Caissons.
In using the ejectors the following mode of working was employed. (See Fig. 48.) A sort of sump or hollow in the ground was formed, and into this the water from an overhead tank was allowed to flow in any quantity desired, and mixed with the more solid material excavated. A man who held the end of the flexible hose attached to the ejector manipulated the same in such a manner that a certain amount of the air inside the chamber was allowed to enter into the ejector pipe with the mixture in the sump. This air in escaping carried with it a certain amount of liquid, and forced it out at one of the openings in the caissons above water level. The operation is shown in the cut figured here. It is somewhat puzzling at first to understand why the air pressure, which is only just equal, or only slightly exceeding that which is due to the head of water outside, should lift and discharge a quantity of semi-solid material at a level above the water; the explanation will, however, lie found in the fact that the velocity of the air due to this pressure is very much higher than that of water flowing under the same head, and that it carried mechanically along with it as much of the fluid as it could convey. Of course the man who manipulated the end of the hose in the sump had to feel his way into the solution of this problem, and had to vary the quantities or proportions of air and liquid according to the requirements of the moment. Outside the caisson the flow from this ejector was not continuous, but in gulps, like that from a single plunger pump, now in a large mass, now in a thin stream, and at times nothing at all.
As soon as all the soft material had been removed, and a bed of stiffer clay reached, the ejector
