ific gravity. It
follows that for a specifically heavy material a greater centrifugal
force will be required; that is to say, the elevator will have to be
higher speeded than in dealing with a lighter material.

[Illustration: FIG. 15.--Mineral Elevator, upper terminal.]

Elevator buckets must be varied according to the nature of the
material; for instance, shallow buckets will be found best for a soft
and clinging material such as flour, moist sugar, sand, small coal,
&c., while for a hard or semi-hard body such as wheat, coal, &c.,
deeper buckets are preferable. On account of their lower speed,
elevators for specifically heavy material require much larger buckets
and chains than grain elevators of the same bulk capacity. The most
economical form of elevator is fitted with a continuous chain of
buckets. Such elevators may be constructed to carry either grain or
minerals. The advantages are greater capacity than an ordinary
elevator of the same dimensions and a more uniform delivery; moreover,
smoother running is secured, since the buckets being close together
need not plunge intermittently through the contents of the
elevator-well.

_Intermittent Conveyors._--The elevators we have been considering,
whether used for carrying and distributing coal or grain, have this in
common, that they raise material from a lower to a higher level, so to
speak, in a continuous stream, the continuity being broken only by the
short spaces between the buckets. In the continuous bucket type indeed
the stream of material is practically, if not absolutely, continuous. In
all these cases the elevator is fed with the material in a continuous
stream, and by some mechanical means; whether by band, worm or shoot, is
immaterial. Elevators of a somewhat different and more substantial
construction may be and are often used for handling filled sacks,
barrels, carcases of animals and other bulky objects, which cannot be
delivered in a uniform stream, but may have to be conveyed by the
elevator intermittently. The ordinary buckets used for grain or coal are
replaced by other appliances for gripping and holding the object to be
raised from a lower to a higher level, but in principle these appliances
are essentially elevators.

[Illustration: FIG. 16.--Mineral Elevator, lower terminal.]

Another kind of elevator, known as a _lift_ or _hoist_, is used in mines
and quarries and in serving blast furnaces. This is an