PRINCIPLE OF THE SCREW. 19 



So different in appearance are the screw and the inclined 

 plane, that very few people would realise the fact that the screw 

 is nothing but an inclined plane wound round a cylinder, or 

 rather, is a circular inclined plane. The ordinary corkscrew 

 is a good example of this principle, the cylinder being but an 

 imaginary one. 



Now, if the screw be turned round, it is evident that force is 

 applied just on the principle of the wedge, and this principle is 

 well shown in the various screw-presses, of which the common 

 linen-press is a familiar example, as was the original printing- 

 press, which still survives as a toy for children. 



We all know the enormous force exerted by screws when 

 working in wood, and how, when the screw- driver is turned in 

 the reverse direction, the instrument is forced backwards, 

 though the operator is leaning against it with all his weight. 

 In fact, a comparatively small screw, if working in hard wood 

 or metal, so that the threads could not break, could lift a heavy 

 man. 



Substitute water for wood or metal, and the result would be 

 the same in principle, though the resistance would be less. As 

 the loss of power by friction would prevent a large vessel from 

 being propelled by a stern oar moved like a scull, the idea was 

 invented of applying the same kind of power by a large screw, 

 which should project into the water from the stern of the 

 vessel. This modification, moreover, would have the advantage 

 of forcing the vessel forward when the screw was turned from 

 left to right, and drawing it back when turned in the opposite 

 direction, whereas the sculling oar would only drive it forward. 



The principle was right enough, but there was at first a great 

 difficulty in carding it out. Firstly, several turns of a large 

 screw were used, and were found to need power inadequate to 

 the effect. Then the screw was reduced to four separate blades, 

 and now only two are used, as shown in the illustration, 

 these saving friction, being equally powerful for propulsion, and 

 running less risk of fouling by rigging blown overboard or 

 other floating substances. 



So much for Art. Now for the same principle as shown in 

 Nature, of which I can take but a very few instances. 



The first and most obvious example is that of the Fish-tail, 

 which any one may observe by watching ordinary gold fish in 



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