140 THE RAISING OF THE DRY-DOCK DEWEY. 
were also fitted with cut-out valves at the tops. These manhole covers 
were placed in position by divers. It was considered hopeless to attempt 
to introduce compressed air through the vent pipes particularly in the end 
pontoons as they were badly corroded and leaky, and furthermore this 
method was considered somewhat hazardous due to the fact that the flow 
of water from the tanks could not have been accurately determined as it 
would have been through the drainage system, and there would have been 
danger of damaging the tanks and bulkheads from excessive pressure. The 
method of introducing compressed air through manhole covers enabled the 
divers at all times to determine with absolute accuracy the action of the air 
and water, as the water could be plainly felt coming out the ejection pipes. 
For the first plan of raising the dock, the 20 manhole covers were in- 
stalled and all vent pipes plugged tight as far as possible. A large amount 
of work was involved in doing this as in some cases whole sections of corroded 
pipe had to be removed or renewed by divers. In all, about 1,100 feet of 
piping were renewed. These pipes could not be made completely tight 
owing to the fact that some were leaky where they passed through tanks 
Nos. 8, 12, and the self-docking tanks, which places were inaccessible to the 
divers. However, they were made sufficiently tight to retain compressed 
air without excessive leakage. 
Fig. 1, Plate 72, shows the starboard side under control and the floats, 
etc., at work. Before introducing compressedair a hose fromthe accumulator 
was led to the floor of the dock. The pressure in this hose was increased 
until the air just came out at the end and such pressure wasnoted. A 
number of hose connections were then made to the various manhole covers 
and air was introduced ata pressure of 2 pounds inexcess of the pressure 
already determined of the head of water outside. These connections were 
all to tanks on the low side, but it was found that the air went at once to the 
tanks on the high side. This was probably due in part to leakage through 
the vent pipes in the tops of the tanks, but it was mainly due to the fact that 
the tank bulkheads proved to be non water-tight at the tops, allowing air 
to pass freely from one tank to another. In one case the air went through 
5 bulkheads, one of them being the center-line bulkhead which had no pipes 
whatever running through it. In all cases the air forced the water out of 
the tanks on the high side first, through the drain piping and closed valves, 
lifting the high side only. This non water-tightness of bulkheads at the tops 
was not known before attempting to introduce the compressed air, there 
being nothing in the plans and specifications to show it. This method of 
lifting the dock was therefore proved to be impracticable and even dangerous 
from the excessive pressure that would have been produced in the tanks on 
