In barrel-pier bridges, or with boats having strong gunnels and frames, each balk bears on both gunnels of adjacent piers; in weak boats they bear on a central beam supported on the keel.
The roadway of floating bridges is similar to that already described.
Boats.—Open boats should not be immersed deeper than within one foot of the gunnel, and a still larger limit of safety will be required in rough water or a violent current. They should be placed "stem on" in the bridge so as to point against the current, and slightly down by the stern; if the river be tidal, they alternate stem and stern. Few boats, except heavy barges, are strong enough for balks to rest on their gunnels. Use a central transom resting on the thwarts and blocked up from underneath to bring the weight directly on the keel.
Buoyancy.—This may be determined by loading the boat with unarmed men to such a depth as is considered safe. For bridging purposes the number of men multiplied by 160 gives the available buoyancy in pounds.
If the number of men be divided by four, the result will be the central interval in feet at which the boats may be placed in the bridge to carry infantry in fours crowded.
RAFTS OF CASKS OR BARRELS.
Buoyancy of Casks.—The actual buoyancy is given by the formula 5c2l-W, in which c is the circumference of the cask in feet half-way between the bung and the extreme end; l is the length in feet, exclusive of projections, measured along a stave; and W is the weight of the cask in pounds.
The available buoyancy for bridging purposes may be taken at 9/10 of the actual buoyancy.
A safe buoyancy may be obtained by multiplying the content in gallons by 8⅓.
Knowing the buoyancy of one cask, the number to be united in a raft to sustain any desired load can be readily calculated.