160 HISTORY OF 



observing that water is possessed of an invariable 

 property, which has not hitherto been mentioned; 

 that of always keeping its surface level and even. 

 Winds, indeed, may raise it into waves, or art 

 spurt it up in fountains ; but ever, when left to 

 itself, it sinks into a smooth even surface, of which 

 no part is higher than another. If I should 

 pour water, for instance, into the arm of a pipe of 

 the shape of the letter U, the fluid would rise in 

 the other arm just to the same height ; because, 

 otherwise, it would not find its level, which it in- 

 variably maintains. A pipe, bending from one hill 

 down into the valley, and rising by another, may 

 be considered as a tube of this kind, in which the 

 water, sinking in one arm, rises to maintain its 

 level in the other. Upon this principle all water- 

 pipes depend; which can never raise the water 

 higher than the fountain from which they pro- 

 ceed. 



Again, let us suppose, for a moment, that the 

 arms of the pipe already mentioned may be made 

 long or short at pleasure ; and let us still further 

 suppose, that there is some obstacle at the bottom 

 of it, which prevents the water poured into one 

 arm from rising in the other. Now it is evident, 

 that this obstacle at the bottom will sustain a pres- 

 sure from the water in one arm, equal to what 

 would make it rise in the other; and this pressure 

 will be great, in proportion as the arm filled with 

 water is tall. We may, therefore, generally con- 

 clude, that the bottom of every vessel is pressed 

 by a force, in proportion to the height of the 

 water in that vessel. For instance, if the vessel 



