ON HYDRAULIC MACHINES. 251 



required for elevating the quantity raised by such a machine, appears 

 from calculation to correspond to a velocity about twice as great as the 

 actual relative velocity. While the water is principally supported by 

 the friction of the rope, its own cohesion is amply sufficient to prevent 

 its wholly falling, or being scattered, by any accidental inequality of the 

 motion. (Plate XXII. Fig. 299.) 



The lateral friction of water has been applied in a very simple manner 

 by Venturi* to the draining of land by means of a stream which runs 

 through it, allowing the stream to acquire sufficient velocity to carry it 

 over an inclined surface, and to drag with it a certain portion of water 

 from the lowest part of this surface : but the quantity of water raised 

 in this manner must be very inconsiderable, and the loss of force by fric- 

 tion very great. 



A system of spiral pipes may be placed in the plane of a wheel, receiving 

 the water at its circumference, and raising it by degrees, as the wheel turns, 

 towards the axis, where it is discharged ; the motion of the wheel being 

 usually derived from the same stream which supplies the pipes : but the 

 height to which the water is raised by this machine is very small in 

 proportion to its bulk. A single pipe wound spirally round a cylinder 

 which revolves on an axis in an oblique situation, has been denominated 

 the screw of Archimedes,t and is called in Germany the water snail. Its 

 operation, like that of the flat spiral, may be easily conceived by imagining 

 a flexible pipe to be laid on an inclined plane, and its lower part to be 

 gradually elevated, so that the fluid in the angle or bend of the pipe may 

 be forced to rise ; or by supposing a tube, formed into a hoop, to be rolled 

 up the same plane, the fluid being forced by the elevation of the tube 

 behind it to run as it were up hill. This instrument is sometimes made by 

 fixing a spiral partition round a cylinder, and covering it with an external 

 coating, either of wood or of metal ; it should be so placed with respect to 

 the surface of the water as to fill in each turn one half of a convolution ; 

 for when the orifice remains always immersed, its effect is much dimi- 

 nished. It is generally inclined to the horizon in an angle of between 45 

 and 60 degrees : hence it is obvious that its utility is limited to those cases 

 in which the water is only to be raised to a moderate height. The spiral 

 is seldom single, but usually consists of three or four separate coils, forming 

 a screw which rises slowly round the cylinder. (Plate XXII. Fig. 300, 

 301.) 



An instrument of a similar nature is called by the Germans a water 

 screw ; it consists of a cylinder with its spiral projections detached from 

 the external cylinder or coating, within which it revolves. This machine 

 might not improperly be considered as a pump, but its operation is pre- 

 cisely similar to that of the screw of Archimedes. It is evident that some 

 loss must here be occasioned by the want of perfect contact between the 



* Prop. 9. 



t Vitruvius, 1. 10, c. 11. Pitot, Hist, et Mem. de Paris, 1736, p. 173, H. 110. 

 TSuler, Nov. Com. Petr. v. 259. Hennert, Dissertation sur la vis d'Archimede, 

 Berl. 1767. Pattu, Journal des Mines, 1815, xxxviii. 321. Gregory's Mechanics, 

 ii. 343. 



