The “Ponton” is another kind of ferry boat.
The larger sized craft are raised forward and aft to facilitate the
passage. The bow and stern, which are broad and flat, carry a movable
part called the “Koebrug” (cow bridge) to facilitate taking horses,
cattle and carts on board. These movable outside bridges are lowered
and raised by two balanced levers, one on each side. By bearing down
on them, the fore bridge is raised a little above the horizontal.
The lever is then made fast. After crossing the river, the bridge is
dropped and rests on the approach. The slope of the bridge must not be
too great, nor should the movable part be too long, otherwise it would
be unmanageable.
It follows from what precedes, that there exists, for a good approach,
a certain relation between the slope of the ramp, the length of the
movable bridge and the draught of the ferry-boat.
As a general rule and for ordinary pontoons, the most favorable slope
for the ramp is 1 in 8.
An easier slope makes too sharp an inclination for the movable bridge;
a steeper slope, which would make the bridge more nearly horizontal,
would make, on the other hand, too difficult an approach for wheeled
vehicles.