LECTURE XXVIII.

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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 velocityof the rope, and on this supposition we might calculate its depth on the ropeby 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 inthis case with the results of direct experiments ; for the friction required forelevating the quantity raised by such a machine, appears from calculation tocorrespond 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 beingscattered, by any accidental inequality of the motion. (Plate XXII. Fig .S 99 .)

The lateral friction of water lias been applied in a very simple manner byVenturi 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 inclinedsurface, and to drag with it a certain portion of water from the lowest partof this surface : but the quantity of water raised in this manner must be veryinconsiderable, and the loss of force by friction very great.

A system of spiral pipes may be placed in the plane of a wheel, receivingthe 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 beingusually derived from the same stream which supplies the pipes: but theheight 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 thescrew 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 aflexible pipe to be laid on an inclined plane, and its lower part to be gra-dually elevated, so that the fluid in the angle or bend of the pipe may beforced to rise ; or by supposing a tube, formed into a hoop, to be rolled upthe same plane, the fluid being forced by the elevation of the tube behind itto run as it were up hill. This instrument is sometimes made by fixing aspiral partition round a cylinder, and covering it with an external coating,either of wood or of m?tal; it should be so placed with respect to the surface