132 HISTORY OF 



.science, which are as well confii-med by ex|)eiiment, as rendered 

 universal by theory. It would, indeed, be unpardonable, while 

 discoiu-sing on the properties of water, to omit giving some ac- 

 count of the manner in which it sustains such immense bulks, 

 as we see floating upon its soft and yielding surface ; how some 

 bodies, that are known to sink at one time, swim with ease, if 

 their surface be enlarged ; how the heaviest body, even gold it- 

 self , may be made to swim upon water : and how the lightest, 

 such as cork, shall remain sunk at the bottom; how the pouring 

 in of a single quart of water, will burst a hogshead hooped with 

 iron : and how it ascends, in pipes, from the valley, to travel 

 over the mountain ; these are circumstances that are at first sur 

 prising ; but, upon a slight consideration, lose their wonder. 



' In order to conceive the manner in which all these wonders 

 are effected, we must begin by observing that water is possessed 

 of an invariable property, which has not hitherto been mentioned, 

 that of alvvays keeping its surface level and even. Winds, in- 

 deed, 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 one part is higher than another. If I should pour water, 

 forinstance, into the arm of a pipe of theshape of the letter U, the 

 fluid would rise in the other arm just to the same height ; be- 

 cause, otherwise, it would not find its level, which it invariably 

 maintains. A pipe bending from one hUl 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 proceed. 



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 pouring into one arm, 

 from rising in the other. Now it is evident, that this obstacle 

 at the bottom will sustain a pressure from the water in one arm, 

 equal to what would make it rise in the other ; and this pressure 



) In the above sketch, the manner of demonstrating- used by Monsieur 

 D'Alembertis made use of, as the most obvious, and the most satisfactory. 

 Vide Essai sar, &c. 



