HYDROSTATICS. 



strument was brought up, stood eight 

 inches high on the rod. The rod there- 

 fore had been forced eight inches into 

 the water in the cylinder, when at the 

 deepest. The pressure upon the rod 

 was about thirteen hundred pounds ; 

 the surface of the end of the rod about 

 one-ninth of the surface of the water 

 in the cylinder, and the cylinder two 

 feet long; the water must therefore 

 have been forced by the rod into a 

 space less than its whole bulk when 

 uncompressed by ^f^, or one twenty- 

 seventh part of that bulk. 



That watery fluids have some elasti- 

 city is indeed so plainly proved by every 

 day's experience, and by simple facts, as 

 to occasion some wonder at. the contrary 

 ever having been asserted upon the 

 authority of any one experiment, espe- 

 cially when that was of a somewhat 

 complicated nature, and in itself far 

 from conclusive. The common play of 

 making ducks and drakes, that is, 

 throwing a flat stone in a direction 

 nearly horizontal against a surface of 

 water, and thus making it rebound, 

 proves the water to be elastic ; and a 

 musket-ball when so fired flies up in 

 like manner, after striking the water. 

 But you have only to pour water into 

 an empty basin to be convinced of its 

 elasticity ; the first water that falls 

 sparks about, flying up from the ba- 

 sin, and then what falls on the surface 

 of the water which has been poured in 

 will not fly so much up, because the 

 water is much less elastic than the 

 basin; and on a glass it will fly still 

 more, glass being the most elastic body 

 we know. But a piece of suet or putty, 

 or any other non- elastic body, will not 

 rebound even from glass. 



CHAPTER II. 



Fundamental Principle of equal 

 Pressure. 



ALL the particles of fluids are so con- 

 nected together, that they press equally 

 in every direction, and are equally 

 pressed upon : each particle presses 

 equally on all the particles that sur- 

 round it, and is equally pressed upon 

 by these ; it equally presses upon the 

 solid bodies which it touches, and is 

 equally pressed upon by those bodies. 

 From this, and from their gravity, it 

 follows, that when a fluid is at rest, 

 and left to itself, all its parts rise or 

 fall, so as to settle at the same level, no 

 part standing above or sinking below the 



rest. Hence if we pour water or any 

 other liquid into a tube (or pipe) bent 

 like a U, it will stand at the veiy same 

 height in both limbs. Nor does it make 

 any difference if one limb is wider than 

 the other. For suppose we knock off the 

 bottoms of a quart bottle and of a phial, 

 and plunge them upright in a trough 

 of water, A B C D \fig. 1 .) ; the water 

 will enter both the phial and the bottle, 

 and stand at the same level in both, 

 being the same with the level of the 

 water, F G, outside, the glass, or of the 

 water in the trough before the bottle 

 and phial were placed in it. Suppose 

 we join the bottoms of the two by a 

 tube, K, passing from one to the other 

 in the water, and inclosing so much 

 water; this will make no difference in 

 the level of the water either in the 

 bottles or in the trough generally. So 

 if we solder this connecting tube to 

 the two upright ones, so as to make 

 the joinings water-tight, and thus to 

 form one vessel, H K, this can make 



no difference on the level, F G, of the 

 water : then, if we remove the vessel 

 thus formed from the trough, the water 

 must stand in it exactly as it did when 

 in the trough, because it is manifestly 

 impossible that it should make any 

 difference to the water inside the bottle, 

 whether there be water on the outside, 

 or only air; and the water will stand 

 as high in the wide bottle as in the 

 narrow phial. In like manner, if, in- 

 stead of filling the bottles by plunging 

 them in the full trough, we pour water 

 into them when empty, and standing in 

 the empty trough, and at the same 

 time we pour water into the trough, 

 the water will stand equally high in 

 both bottles : and so if we only pour 

 it into the bottles, and not into the 

 trough at all, or into the bottles with- 

 out any trough ; because it can make 

 no difference to the water inside the 

 glass, whether there be any outside 

 or not, there being no communication 

 whatever between the inside and out- 

 side. Generally, then, and in every 

 case, if there be two tubes or limbs 

 of a tube connected together, how- 



