48 PRESSURE OF FLUIDS. 



the nature, gravity, pressure, and motion of fluids, in general, 

 and of the methods of weighing solids in them, is called hy- 

 drostatics. The non-elastic fluids are said to be incompres- 

 sible, not because they are absolutely so, but because their 

 compressibility is so very small as to make no sensible differ 

 rence in calculations relative to their several properties. It 

 has been found that water will find its way through the pores 

 of gold, rather than suffer itself to be compressed into a 

 smaller space. At Florence, a celebrated city in Italy, a 

 globe made of gold was filled with water, and closed so ac- 

 curately that none of it could escape. The globe was then 

 put into a press, and a little flattened at the sides : the con- 

 sequence of which was, that the water came through the fine 

 pores of the gulden globe, and stood upon its surface like 

 drops of dew. It was concluded at that time that water was 

 incompressible. Later experiments, however, have shown, 

 that those fluids which were esteemed incompressible, are, 

 in a very small degree, as, perhaps, one part in twenty 

 thousand, capable of compression. 



Fluids are subject to the same laws of gravity as solids ; 

 but their want of cohesion occasions some peculiarities. The 

 parts of a solid are so connected as to form a whole, and their 

 weight is concentrated in a single point, called the centre 

 of gravity ; but the atoms of a fluid gravitate independently 

 of each other. It is on this account that water always finds 

 its level ; for when any particle accidentally finds itself ele- 

 vated above the rest, it is attracted down to the level of the 

 surface, and the readiness with which water yields to the 

 slightest impression, will enable the particle by its weight to 

 penetrate the surface and mix with it. The particles of a 

 fluid acting thus independently, press against each other in 

 every direction, not only downwards but upwards, and late- 

 rally or sideways, and in consequence of this equality of 

 pressure, every particle remains at rest in the fluid. If you 

 agitate the fluid you disturb this equality of pressure, and it 

 will not rest till its equilibrium or level is restored. "The 

 pressure downwards is the effect of gravity, and if there were 

 no lateral pressure, water would not run out of an opening 

 on the side of a vessel. The lateral pressure proceeds entirely 

 from the downward pressure, or the weight of the liquid 

 above ; and consequently the lower an orifice is made in a 

 yessel, the greater will be the velocity of the water rushing 



