HYDROSTATICS. 



limb ; turn the stop- cock F, and the 

 water will sink a little in the wide limb, 



fig- 4. 



and rise to the same level, I, in the 

 narrow one : in like manner, while the 

 obstruction remains, you may raise 

 the water to I, and if it be removed, it 

 will fall in B D, while in A K it will 

 rise till it is equally high in both. So 

 that the small quantity I B balances 

 the large quantity GAEH, because 

 both press on the same space at F, and 

 both are of the same height : for whe- 

 ther the two volumes of water IDF and 

 G E F press on each other, or on some 

 surface placed between them at F, can 

 make no difference ; if they balance 

 each other it is because they press 

 equally : they therefore press "equally 

 on the same plate or other solid body 

 placed between them ; and in like man- 

 ner two volumes of water quite uncon- 

 nected and standing upon different bo- 

 dies, whatever the bulk of water may 

 be, press upon an equal extent of sur- 

 face, with a weight proportioned to the 

 height, and not at all to the bulk of the 

 water. Hence the general rule for esti- 

 mating the pressure of any fluid is, to 

 multiply the height of the fluid by the 

 extent of the surface on which it stands, 

 and the product gives a mass which 

 presses with the same weight as the 

 fluid standing on that surface, however 

 shallow it may be, provided any portion 

 is supported at the height by a tube. 

 If A B (fig. 5.), a tube twenty feet high, 

 fiff.5. 



and one inch in bore (or diameter), be 

 filled with water, and plunged into a 

 space, C D E F, three feet square and 

 half an inch deep, likewise filled with 

 water, there will only stand in A B and 

 C D F E together thirty- eight pounds 

 troy of water; yet this water will 

 press in all directions, downwards, up- 

 wards, and sideways, with the same 

 force as if the whole space A D C B 

 were filled with water, that is, as if 

 there were five tons of water standing 

 on B C. 



The equal pressure of fluids in eveiy 

 direction, is illustrated by many very 

 simple experiments. If a weight of a 

 pound or two is placed upon a thin 

 plate of glass, it breaks through the 

 glass immediately, because all the 

 pressure is from above ; but if the 

 glass is laid on a flat surface sufficiently 

 strong to support it, the weight will 

 not break the glass, provided it be laid 

 gently on, because there is a resistance 

 on the under side equal to the pressure 

 on the upper. If the glass plate is made 

 the bottom of a cistern, and a sufficient 

 weight of water poured in, it will break 

 the glass in the same way ; but if the 

 plate be placed in a cistern, however 

 deep, so that there may be water on 

 both sides of it, however thin the glass 

 may be, it will not be broken. Sup- 

 pose the glass is a foot square, and 

 is placed twelve feet deep, and is as 

 thin as a piece of the finest cambric 

 paper, so that the weight of a few 

 grains would break through it in the 

 air ; it will support a weight of twelve 

 cubic feet of water, or nearly seven 

 hundred weight, without being broken, 

 crushed, or cracked ; or if the cistern 

 be filled \vith mercury, the film of glass 

 will support a weight of above four 

 tons and a half without any injury. 

 This could only happen by the pressure 

 of the fluid being exactly the same up- 

 wards and downwards, and in all di- 

 rections. So if a force is applied to 

 the water for the purpose of pressing 

 it, how r ever great the force may be 

 which is thus conveyed through the 

 liquid to the solid plunged in it, though 

 that solid be an egg, or an egg-shell 

 filled with water, or a piece of the 

 finest spun glass, or a spider's web, the 

 shape will remain wholly unchanged by 

 the pressure applied. This is some- 

 times illustrated still further by a cu- 

 rious experiment. An egg and a piece 

 of very soft wax are placed in a blad- 

 der filled with water, and this is placed 



