h, and the
installation acquires a new character, becoming, in point of fact, a
moving bridge which is drawn across its supports and fits into the
grooves in the wheels surmounting the latter. The carriage or truck
may be constructed on the plan adopted for the building of the longest
type of modern bogie carriages for ordinary railways, the tensile
strength of steel rods being largely utilised for imparting rigidity.
We now find that instead of a railway we have the idea of what may be
more appropriately called a "wheelway". The primitive application of
the same principle is to be seen in the devices used in dockyards and
workshops for moving heavy weights along the ground by skidding them
on rollers. Practically the main precaution observed in carrying out
this operation is the taking care that no two rollers are put so far
apart that the centre of gravity of the object to be conveyed shall
have passed over one before the end has come in contact with the next
just ahead of it.

The "wheelway" itself will be economical in proportion as the length
of the rigid carriage or truck which runs upon it is increased. The
carrying of cheap freight will be the special province of the
apparatus, and it will therefore be an object to secure the form of
truck which will give, with the least expense, the greatest degree of
rigidity over the longest stretch of span from one support to another.
Some modification of the tubular principle will probably supply the
most promising form for the purpose. The hope of this will be greatly
enhanced through the recent advances in the art of tube-constructing
by which wrought-iron and tough steel tubes can be made quite seamless
and jointless, being practically forged at one operation in the
required tubular shape.

For mining and other similar purposes, the long tubal "wheelway"
trucks of this description can be drawn up an incline at the loading
station so as to be partially "up-ended" in position for receiving the
charges or loads of mineral or other freight. After this they can be
despatched along the "wheelway" on the closing of the door at the
loading end. In regard to the mode of application of the power in
traction, the shorter-distance lines may serve their objects well
enough by adopting the endless wire-rope system at present used on
many mining properties.

But it is found in practice that for heavy freight this endless cable
traction does not suit over distances of more than abou