78 A HISTORY OF 



much improved, rather belongs to experimental than to natural histo- 

 ry. However, 1 will take leave to mention some of the most striking 

 paradoxes in this branch of science, which are as well confirmed by 

 experiment, as rendered universal by theory. It would, indeed, be 

 unpardonable, while discoursing on the properties of water, to omit 

 giving some account of the manner in which it sustains such im- 

 mense bulks as we see floating upon its soft and yielding surface : 

 how some bodies, that are known f o sink at one time, swim with ease, 

 if their surface be enlarged : how the heaviest body, even gold itself, 

 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 sin- 

 gle 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 surprising ; but, upon a slight 

 consideration, lose their wonder. 



* In order to conceive the manner in vnich all these wonders are 

 effected, we must begin by observing tnat water is possessed of an in- 

 variable 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 



{'ust to the same height ; because, otherwise, it would not find its 

 evel, which it invariably 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 higheV than the fountain 

 from which they proceed. 



Again, let us suppose for a moment, that the arms of the pipe al- 

 ready 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 pressure 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 pro- 

 portion to the height of the water in that vessel. For instance, if the 

 vessel filled with water be forty feet high, the bottom of that vessel 

 will sustain such a pressure as would raise the same water forty feet 

 high, which is very great. From hence we see how extremely apt 

 our pipes, that convey water to the city, are to burst ; for descending 

 from a hill of more than forty feet high, they are pressed by the wa- 

 ter contained in them, with a force equal to what would raise it to 

 more than forty feet high ; and that this is sometime^ able to burst a 

 wooden pipe, we can have no room to doubt of. 



In the above sketch, the manner of demonstrating used by Monsieur D'Alemrwrt k 

 wade use o as the most obvious, and the most satisfactory. Vide Essai sur, &o. 



