ay in a
succession of separate loads, as happens with single bucket elevators,
furnace hoists, rope and chain haulage, and also in the case of ropeways
and aerial cableways. Some of these devices are of great antiquity,
others are of quite modern origin. The principles of their construction
are simple and easy of understanding, but by variations in the details
of their construction the engineer has adapted these few appliances to
the most varied work. At one end of the scale they may be used for such
light duties as conveying the goods purchased by a customer to the
packers and bringing them back made up into a parcel or for taking his
money to the cashier and returning the change. At the other they are
adopted for handling large quantities of heavy material at a minimum
expenditure of human labour. Coal, for instance, a more or less friable
substance, the value of which is seriously diminished by fracture, may
be mechanically handled with a minimum risk of breakage. The difficult
problem of handling the contents of gas retorts and coke ovens, and of
simultaneously quenching and conveying the glowing material, has been
solved. Perhaps an even more astonishing piece of work is the
manipulation of the iron from the blast furnace; for instance, liquid
metal is drawn from a furnace into pouring pots which in their turn
discharge it to and distribute it over a pig-iron casting machine, which
is practically a conveyor for liquid metal, consisting of a strand of
moving moulds from which the solidified pigs, after cooling in water,
are automatically removed after reaching the loading terminal over the
railway trucks. Certain types of conveyors may be made to combine
efficiently, with their primary work of transport, complex sorting,
sifting, drying and weighing operations.

_Worm Conveyors._--The worm conveyor, also known as the Archimedean
screw, is doubtless the most ancient form of conveyor. It consists of a
continuous or broken blade screw set on a spindle. This spindle is made
to revolve in a suitable trough, and as it revolves any material put in
is propelled by the screw from one end of the trough to the other. Such
conveyors have been used in flour-mills for centuries. The writer has
seen in an East Anglian mill which was over 250 years old disused screw
conveyors, probably as old as the mill, consisting of spindles of
octagonal shape, made of not too hard wood, around which a broken blade
screw was formed by the insert