J28 LECTURE XXVIII. 



feet in a second, and drawing a certain quantity of water up by its friction. 

 It is probable that the water commonly ascends with about half the velocity 

 of the rope, and on this supposition we might calculate its depth on the rope 

 by comparing its relative motion with that of a little river : but the rules, 

 which serve for calculating the velocity of rivers, do not perfectly agree in 

 this case with the results of direct experiments ; for the friction 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 friction 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 propor- 

 tion to its bulk, A single pipe wound spirally round a cylinder which re- 

 volves on an axis in an oblique situation, has been denominated the 

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

 tion, 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 gra- 

 dually elevated, so that the fluid in the angje 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 



