of different densities, the lighter of which could be an immiscible liquid or 
preferably a low-molecular-weight gas such as helium or hydrogen to minimize 
frictional losses. Air should be entirely satisfactory for most conditions and 
pressures, although there may be some loss in efficiency strictly on the basis 
of the high viscosity and density of air at high pressures. Whether the potential 
advantages of the lighter gases outweigh the disadvantages of special handling, 
supply, and cost remains to be proved by trade-off studies and tests. 
barrel 
pressure head 
Figure 64. Test of high-velocity water jet for penetrating hard rock (Cooley and 
Clipp, 1969). 
86 
