Under average conditions the top limit may be placed at
about 45 pounds, the time of working, according to conditions,
varying from four to six hours per shift. In the cases in which
higher pressures might be used, the shifts for the men should be
restricted to two of two hours each, separated by a considerable
interval. As an example of heavy pressure work under favorable
conditions as to ventilation, without very bad effects on the
men, Messrs. Sooysmith & Company had an experience with a work on
which men were engaged in six-hour shifts, separated into two
parts by half-hour intervals for lunch. This work was excavation
in open, seamy rock, carried on for several weeks under about 45
pounds pressure. The character of the material through which the
caisson is being sunk or upon which it may be resting at any time
bears quite largely upon the ability of the men to stand the
pressure necessary to hold back the water at that point. If the
material be so porous as to permit a considerable leakage of air
through it, there will naturally result a continuous change of
air in the working chamber, and a corresponding relief of the men
from the deleterious effects which are nearly always produced by
over-used air.
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