252 LECTURE XXVIII. 



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 considerable depth ; so that if the 

 height of the surface of the water to be raised were liable to any great vari- 

 ations, 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 communicates 

 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 be- 

 comes equivalent to that of the column of w r ater, or of water mixed with 

 air, in the ascending pipe. The air nearest the joint is compressed into a 

 space much smaller than that which it occupied at its entrance, so that 

 where the height is considerable, it becomes advisable to admit a larger 

 portion of air than would naturally fill half the coil, and this lessens the 

 quantity of water raised, but it lessens also the force required to turn the 

 machine. The joint ought to be conical, in order that it may be tightened 

 when it becomes loose, and the pressure ought to be removed from it as 

 much as possible. The loss of power, supposing the machine well con- 

 structed, arises only from the friction of the water on the pipe, and the 

 friction of the wheel on its axis ; and where a large quantity of water is to 

 be raised to a moderate height, both of these resistances may be rendered 

 inconsiderable. But when the height is very great, the length of the spiral 

 must be much increased, so that the weight of the pipe becomes extremely 

 cumbersome, and causes a great friction on the axis, as well as a strain on 

 the machinery : thus, for a height of 40 feet, I found that the wheel 

 required above 100 feet of a pipe which was three quarters of an inch in 

 diameter ; and more than one half of the pipe being always full of water, 

 we have to overcome the friction of about 80 feet of such a pipe, which will 

 require 24 times as much excess of pressure to produce a given velocity, as 

 if there were no friction. The centrifugal force of the water in the wheel 

 would also materially impede its ascent if the velocity were considerable, 

 since it would be always possible to turn it so rapidly as to throw the 

 whole water back into the spoon. The machine which I had erected being 



* Sulzer's Sammlungen Vermischeln Schriften, 1754. Ziegler, Gesellschaft zu 

 Zurich, vol. iii. Nicander, Schwed. Abhand. 1783. 



