344 HYDROSTATICS 



earth resting on the top of the wall changes only slightly with 

 the increase in thickness of the wall. The preceding table 

 shows the proper ratios of thickness to height for vertical walls 

 with various amounts of surcharge. After ascertaining the 

 thickness of the vertical wall required for restraining a sur- 

 charge bank, the form of the wall may be altered to give a 

 battered face or back, or both, in the same way as if the 

 top of the backing were level with the top of the wall. 



HYDROSTATICS 



DEFINITIONS AND GENERAL 

 PRINCIPLES 



Hydrostatics treats of the equilibmtm of liquids" and of their 

 pressures on the walls of vessels containing them and on sub- 

 merged surfaces. 



Liquid Bodies. A liquid is a body whose molecules change 

 their relative positions easily, being, however, held in such a 

 state of aggregation that, although the body can freely change 

 its shape, it retains a definite and invariable volume, provided 

 the pressure and temperature are not changed. Water and 

 alcohol are examples of liquid bodies. 



A perfect liquid is a liquid without internal friction; that is 

 one whose particles can move on one another with absolute 

 freedom. On account of this characteristic property, a perfect 

 liquid offers no resistance to a change of form. 



A viscous liquid is a liquid that offers resistance to rapid 

 change of form on account of internal friction, or viscosity. 

 Tar, molasses, and glycerine are examples of viscous liquids. 



All liquids are more or less viscous. For the purposes of 

 hydrostatics, however, water, which is the liquid mainly dealt 

 with, may be treated as a perfect liquid, its viscosity at ordinary 

 temperatures being too small to be taken into account. 



Compressibility. All liquids offer great resistance to change 

 in volume; that is, they can be compressed but little. Under 

 the pressure of 1 atmosphere (about 14.7 Ib. per sq. in.), water 

 is compressed about rtifans of its original volume. For engineer- 

 ing purposes it may be assumed that water is incompressible. 



