ATMOSPHERIC PRESSURE 



39 



A dome d contains air. At each downstroke of p 

 water forced into e rises a little way into d. The air 

 in d, being elastic, drives out the water during an up- 

 stroke of p, and this keeps up a more even flow in the 

 pipe e. The dome is not strictly needed, but is gener- 

 ally used on force pumps to make the stream steady. 



Water may be forced any distance if the pump is 

 strong enough to do it, though great force may have 

 to be used. Windmills and hot-air engines are commonly 

 used to fill small tanks, while city water-supply systems 

 make use of enormous pumps run by steam. Fire engines 

 are only steam force pumps. 



49. Siphon. A siphon is a bent tube used for lifting 

 fluids quietly from one vessel to another. It makes use 

 of two forces 

 gravity and atmos- 

 pheric pressure. 



Fig. 25 shows 

 a simple siphon. 

 The bent tube abc 

 is filled with liquid 

 from the vessel 

 w, and allowed to 

 hang so that the end c is lower than the surface of the 

 water (x) in m. In this position gravity acts upon the 

 liquid in both arms of the tube, but more strongly 

 upon be than bx. because c is lower than x ; thus the 

 water will run downward out of be. This tends to 

 cause a vacuum in the tube at 5, and atmospheric 

 pressure forces water up ab to fill this vacuum. Of 



FIG. 25 



