250 



THE SIPHON. 



upon one another is in most cases very simple, if we 

 recollect that in a liquid the pressure increases from 

 above downwards, and decreases from below upwards; 

 that in the same horizontal layer the pressure is every- 

 where the same; and that at any given point in a liquid, 

 in consequence of the great mobility of the liquid 

 molecules, the pressure is the same in all directions; 

 further, that in gaseous bodies the pressure likewise 

 increases from above downwards, but that, in conse- 

 quence of the low specific gravities of gases, the difference 

 of pressure at different altitudes is very inconsiderable 

 unless the difference of the altitudes is very great. 

 The difference of pressure at the top and base of a 

 mass of air which has a height of several centimetres, or 

 even of several metres, is so small, that no sensible error 

 is committed if the difference of pressure is altogether 

 neglected, and the pressure at the base assumed to be 

 the same as at the top. 



Fig. 171 represents a tube, abed, bent twice at 



FIG. 171 (% real size). 



right angles and open at both ends. Suppose the tube 

 to be filled with water and its longer branch immersed 

 in water so that the other end d is at the same height 



