TRANSMISSION OF PRESSURE. 161 



A level determines only the horizontality of a line ; a 

 surface cannot be said to be horizontal unless the level 

 has always the bubble in the centre, in whatever posi- 

 tion the instrument may be placed upon the surface. 



When pressure is applied at the upper side of a book 

 lying on the table, or at one end of a stick the 

 3ther end of which rests upon the ground, the pressure is 

 transmitted through the book and along the stick; and 

 f we leave out of consideration the weight of these 

 Bodies, we may say that the table or the ground has to 

 )ear the pressure which is applied to the book or the 

 tick. A mass which consists of a soft substance or 

 i)f small loose particles behaves somewhat differently, 

 y^hen pressure is applied to a lump of moist clay, which 

 ies on the table, the pressure will also be transmitted 



the table, but the particles of the body will at the 

 ame time be pressed laterally, and the lump of clay will 

 ecome flattened. If a mass of flour or sand, con- 

 fined in a paper or linen bag, be strongly pressed 

 r hile it rests upon the table, there will be the same 

 tteral pressure of the particles; and if it be sufficiently 

 reat, the bag will burst at the sides. In a liquid the 

 lltimate particles, having very little mutual cohesion, 

 ; anifest this lateral pressure in the most perfect 



anner. 



Pressure applied to a liquid is transmitted in all direc- 

 ms equally, that is, equal surfaces bear equal pressure. 

 | a bottle filled with any liquid be closed with a tight - 

 ijting cork, so that no space is left between the cork 

 ' d the liquid, a moderate blow upon the cork may burst 

 1e bottle. The pressure upon the cork is transmitted 



1 such a manner that each portion of the interior sur- 



M 



