eater specific gravity, such as
cement, &c., it is advisable to use a smaller pitch than for substances
of lower specific gravity, such as grain. The capacity of a continuous
worm exceeds that of either a paddle or spiral conveyor of the same
diameter, pitch and speed. As regards the relative efficiency of paddle
and spiral conveyors a series of careful tests made by the writer
indicated that, run at a slow speed the paddle worm, but at a high speed
the spiral worm, has the greater efficiency. There is of course a speed
at which the efficiency of both types is about equal, and that is at 150
revolutions per minute for conveyors 4 to 6 in. in diameter.

[Illustration: FIG. 4.--Spiral or Anti-Friction Conveyor.]

The power necessary to drive worm conveyors under normal conditions is
very considerable; a continuous worm of 18 to 20 in. diameter running at
60 revolutions per minute will convey 50 tons of grain per hour over a
distance of a hundred feet at an expenditure of 18-1/2 to 19 H.P. A
material like cement would require rather more power because of the
greater friction of the cement against the blades and the trough.
Delivery from a worm conveyor can be effected at any desired point, all
that is necessary being to cut an outlet, which should preferably be as
wide as the diameter of the worm, because the worm delivers only on its
leading side, and is practically empty on the other side, so that a
smaller outlet might only give exit to a portion of the feed, unless it
was on the leading side.

A special form of worm conveyor is the _tubular_ (fig. 5), which
consists of an iron tube with a continuous spiral fitted to its inner
periphery, or of iron or wooden tubes of square sections fitted with
fixed baffle plates inside. In working it revolves bodily on suitable
rollers. This type is more costly than the ordinary worm conveyors, and
also requires more power. Its efficiency is, moreover, easily impaired
if run at too high a speed, because the centrifugal force asserts itself
and counteracts the propulsion, which in this case is effected by
gravity. Some experiments made in 1868 by George Fosbery Lyster,
engineer of the Liverpool docks, gave convincing results (see _Proc.
Inst. Mech. Eng._, August 1869). The tubular worm conveyor is suitable
where a granular material has to be moved over a comparatively short
distance, say from one building to another on the same level, and where
no bridge is available for the installa