ON HYDRAULIC MACHINES. 329 



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 diminished. It is gene- 

 rally inclined to the horizon in an angle of hetwcen 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 im- 

 properly be considered as a pump, but its operation is precisely similar to that 

 of the screw of Archimedes. It is evident that some loss must here be oc- 

 casioned by the want of perfect contact between the screw and its cover; 

 in general, at least one third of the water runs back, and the machine cannot 

 be placed at a greater elevation than 30° ; it is also very easily clogged by 

 accidental impurities of the water: yet it has been found to raise more water than 

 the screw of Archimedes, when the lower ends of both are immersed to a con- 

 siderable depth ; so that if the height of the surface of the water to be raised 

 were liable to any great variations, the water screw might be preferable to the 

 screw of Archimedes. (Plate XXII. Fig. 302.) 



When a spiral pipe, consisting of many convolutions, arranged either in a 

 single plane, or in a cylindrical or conical surface, and revolving round a 

 horizontal axis, is connected at one end by a watertight joint with an as- 

 cending pipe, while the other end receives during each revolution nearly 

 equal quantities of air and water, the machine is called a spiral pump. It was 

 invented about 1746, by Andrew Wirtz, a pewterer at Zurich, and it is said to 

 have been used with great success at Florence and in Russia : it has also been 

 employed in this country by Lord Stanhope, and I have made trial of it for 

 raising water to a height of forty feet. The end of the pipe is furnished with 

 a spoon, containing as much water as will fill half a coil, which enters the 

 pipe a little before the spoon has arrived at its highest situation, the other 

 half remaining full of air, which conmiunicates the pressure of the column of 

 water to the preceding portion, and in this manner the effect of nearly all 

 the water in the wheel is united, and becomes equivalent to that of the co- 



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