tion of any other kind of
conveyor. Conveyors of this type have, however, come into use for
conveying hard and cutting substances over considerable lengths.
Ordinary worm conveyors are practically debarred from use for such
substances on account of the short life of the intermediate bearings,
which are not necessary with externally supported tubular worms.

[Illustration: FIG. 5.--Tubular Worm Conveyor.]

To sum up, worm conveyors are of the simplest construction and of small
prime cost. The terminals again are much less expensive than those of
most other kinds of conveyors. When the distance to be traversed by the
material is short, the worm conveyor has this advantage, that it is
cheaper than other kinds of conveyors. If it be desired not only to
convey but also to mix two or more materials, such as cement and sand in
a dry state, or poultry food, this appliance is thoroughly well adapted
for the work. On the other hand, there is a grinding action exercised on
any material conveyed, and when hard or cutting substances are handled
the wear and tear on the conveyor blades, trough and bearings is very
great, and the power absorbed by a worm conveyor is a sensible item.

_Band Conveyors._--The inventor of band conveyors for the handling of
grain and minerals was G. F. Lyster, who, as already mentioned, in 1868
carried out exhaustive experiments at the Liverpool docks, where he
established the band conveyor as a grain-handler. For granaries the band
conveyor is an ideal appliance. Its capacity is great, and it can be run
at relatively high speeds with a moderate expenditure of power. The band
conveyor of to-day is an endless belt of canvas or more often
india-rubber with insertion, and when fitted with the usual receiving
and delivery appliances can be used to handle grain from or into
granaries and also to feed bins or sections of a warehouse. The endless
bands run over terminal pulleys, and are also supported on their way by
a series of guide rollers, which are in greater number on the loaded
than on the empty strand. The band is usually run quite flat, except
that at the point or points where the grain is fed on it is slightly
hollowed for a few feet, by means of two curving rolls which are set
obliquely so as to make it trough-shaped. The supporting or guide
rollers are 4 in. to 6 in. in diameter, and are sometimes made of wood,
but more often consist of steel tubes to which spindles with conical end
gudgeons are sec