The continuous or closed spiral worm is illustrated in the Figure. The only difference i between this and the last-mentioned conveyor consists in this, that the screw is not composed of single blades, but is one continuous sheet-iron spiral, which is secured to the spindle at intervals more or less frequent according to the size and capacity of the worm. The most approved method of construction is to cut a narrow spiral groove in the spindle, and to secure the sheet-iron spiral into this groove.