MUTUAL CHANGES COMMON TO ALL MATTER 



59 



before we can gain clearer conceptions, either of the manner 

 in which a gas can resist a liquid, as at A in No. 5, or of the 

 nature of the equilibrium in such a case as No. 3. 



(5) 



Fig. 17. 



(4) 



43. The Equilibrium of Two Liquid Columns in Communi- 

 cation. When a liquid water, for example is at rest in a 

 bent tube of even bore of the shape A (fig. 18), the two surfaces 

 are at the same level, and we may consider any two portions 

 of the vertical columns which are equal in vertical height, 

 as aft and cd, to balance one another that is, the stresses 

 between these two equal quantities of matter and the earth 

 are equal. 



If, however, the tube is of the shape B (fig. 18), the sur- 

 faces are similarly level, although equally long columns, a b 

 and c d, do not contain equal quantities of matter. It is neces- 

 sary to understand why two such columns are in equilibrium 



