necessary is to fit the trough with a sieve which divides it
into an upper and lower deck. The coarser material passes along the
top of the sieve, while the finer coal, sifted out by the
perforations, travels along the bottom of the trough till discharged.
In spite of the gentle propelling action of this conveyor, it has a
thorough sifting action; a perforated plate from 10 to 12 ft. long is
usually sufficient to separate any desired grade, and at a certain
Belgian colliery a conveyor of this type fitted with grading sieves
feeds seven trucks standing in a row, but each on a different siding,
and each taking coal of a different size. This conveyor has been found
useful both as a drying and cooling appliance. Several substances of a
sticky nature, such as moist sugar, which are difficult to deal with
mechanically, can be efficiently handled by the swinging conveyor.

The _gravity_ or _tilting bucket_ conveyor can be used as a combined
elevator and conveyor. It consists essentially of two endless chains or
ropes held at fixed distances apart by suitable bars which are fitted
with small rollers at each end. Every link, or second link, carries a
bucket, and the whole forms an endless chain of buckets. But these
buckets, unlike elevator buckets, which are bolted on to a band or
chain, are free to move on the axis on which they are suspended above
their centre of gravity. When the conveyor is at work the buckets will
always be in an upright position, whether the motion be vertical or
horizontal. Each bucket carries its load to the point at which delivery
is required, where an adjustable tippling device is ready to catch and
tilt the bucket, thus emptying it. This type of conveyor is chiefly used
in connexion with coal stores and boiler houses, where it has undeniable
advantages. For instance, in feeding overhead bunkers a well-designed
gravity bucket conveyor may do the work of (1) a horizontal conveyor in
bringing coal from the railway siding, (2) a vertical elevator in
raising it to the bunkers, and (3) a horizontal conveyor in distributing
it to the respective bunkers. In some cases the returning empty strand
of buckets is used to clear the ashes from under the boilers.

Conveyors of this type run at a mean rate of 40 ft. per minute, and if
it be desired to attain a given capacity the size of the buckets must be
adapted to the increased load as an increase of speed for a higher
capacity is impracti