364 fcUCHOlLb 5 SEAMANSHIP AND NAUTICAL KNOWLEDGE

pressure on the top would be 1 ft X64 Ibs. =64 Ibs ; the pressure on the
bottom would be 2ft.x641bs =128 Ibs; the pressure on each side would be

Fig. 4.
1-5 ft. X64 Ibs.=96 Ibs. per square foot at any point halfway down the
side of the box.
Example.—Required the pressure in tons on the bottom plating of a
ship at a depth of 20 feet, the area of which is 9000 square feet.
The pressure in Ibs. = 64 Ibs. X area X depth.
Pressure=64 ibs. X9000 square ft.x20 ft.~2240 lbs.=5143 tons.
Example.—Required the average pressure on a plate 10 ft. by 4 ft.
suspended vertically in water, the top edge being 20 feet below the
surface.
Area of the plate is 10 ft. X 4 ft. =40 sq. ft.
Mean depth is } (20+30)=25 ft.
Pressure is 64 Ibs. X25 ft. (depth) X 40 sq. ft. (area) =64,000 Ibs.
Example.—The outer bottom is punctured so that a vessel is floating
on her tank tops which have an area of 30 ft. X40 ft. at a depth of 18 ft.
Required the mean upward thrust of the inner bottom in tons.
Pressure=64 Ibs. Xl8 ft. X1200 sq. ft.~-2240 lbs.=617 tons.
Water Pressure Tests.—Tanks are tested by a head of water usually
8 feet high. The sea-cock of the tank is opened to allow the water to
run in, and if the level of the sea surface is higher than the inner bottom
the water will endeavour to reach the same level inside the vessel as
it is* outside. If the difference of level is 8 feet the mean upward thrust
on the inner side of the tank top will be 64 Ibs. X 8 ft. =512 Ibs. per square
foot of area.
The same pressure can be obtained by means of a "stand" pipe,
which is just a, vertical pipe with its bottom end screwed into a hole in