MECHANICS 81 



also emptied of air, either by an exhausting syringe 

 or by the action of the mouth in sucking. When 

 it is thus emptied, the pressure of the atmosphere on 

 the water in A immediately causes it to rise; because 

 there is no longer any pressure within the tube to 

 counteract that of the atmosphere over A, the air in the 

 tube having been removed. Suction, or the action of 

 sucking, essentially consists in the withdrawal of air, 

 followed by changes induced in consequence through 

 atmospheric pressure. The immersed limb of the siphon 

 must be shorter than the other, in order that the 

 resultant pressure of the liquid in the tube, and of the 

 atmospheric pressure may act in the direction CDE. 

 Were both the limbs of the tube of the same length, the 

 atmospheric pressure at either end being equal, the water 

 would then simply fall back into the vessels A and B. 



In a boy's squirt the principle of suction is brought 

 most simply into operation, as also in the common house- 

 hold pump. In the common pump there is a valve at 

 the bottom of the space in which the piston works, and 

 this opens and allows the water to ascend (through the 

 tube which dips down into it) up to that space and so fill 

 the partial vacuum produced by the ascent of the piston. 

 Another valve in the piston opens to allow water to 

 ascend towards the spout, together with any air which 

 may be left to escape, while the lower valve simul- 

 taneously closes, and so prevents the re-descent of the 

 water previously raised. 



It was long supposed that this ascent of water was due 

 to the production, by pumping, of an absolute vacuum, 

 which being a thing Nature abhorred, water rose spon- 

 taneously to fill it. Even Galileo thought it was due to 

 an attraction exercised on water by the piston. A deep 

 well at Florence having failed to draw water, the 



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