sequence of the rotation of the tube, or bends, within which it is confined. Thus the water continually proceeds towards the upper part of the machine, from whence it is discharged into a reservoir placed to receive it It is shown, by writers on hydraulics, that this machine cannot raise water when the angle which a line drawn centrally on the spiral bends makes with planes parallel to the base of the cylinder is greater than the angle which the latter makes with the horizon ; and it is recommended that, in practice, the angle which the axis of the cylinder makes with the horizon should be between 90 and 60 degrees. Such a machine is particularly useful when the water is mixed with gravel, weeds, and the like, which would spoil the action of a common pump.
For computations concerniug the force requisite to turn the tnachine, and the quantity of water which it will raise in a given time, see Gregory's `Mechanics; vol. ii.
A machine consisting of a pipe wound spirally about the surface of a cylinder, or cone, which is made to revolve about its axis when the latter is in a horizontal position, is called a spiral-pump. At one
extremity of the spiral, water and air in nearly equal quantities being allowed to enter, the former will, in consequence of the revolution, be forced up an ascending pipe which may be attached at the other extremity.
The Archirnedian screw has been occasionally employed in modern times to raise water from docks, basins, &c.; and it might be used to raise globular bodies, as cannon balls, from one level to a higher, as from the hold of a ship to the top of a wharf. A similar machine, having the spiral detached from and revolving within the cylinder which is about it, is also used. It is said to raise more water than the usual screw, but it cannot be elevated at a greater angle with the horizon than 30°, and its action is more easily impeded by the sand or gravel which is frequently mixed with the water.
For the application of the screw to navigation, we must refer to STEAM Nes warioN.