4 THE SEQUENCE OF EVENTS 



condition and the products at high temperatures and pressure. This 

 clearly defined rapidly advancing discontinuity is known as a "detona- 

 tion wave," and travels with a velocity of several thousand meters per 

 second. On the other hand, the chemical reaction may take place more 

 slowly and be unable to keep up with the advancing phj^sical disturb- 

 ance of pressure and particle motion which it causes. The final reaction 

 state is then reached more gradually and there is not a well defined 

 boundary. This more gradual process is called "burning," although 

 the rate at which it occurs may still be high. 



The two types of disturbance, detonation and burning, correspond 

 closely to the two major classifications of military explosives: high ex- 

 plosives such as TNT, which detonate with large and rapid evolution 

 of energy and are used for destructive purposes in bombs, depth charges, 

 torpedo warheads; and "propellants" such as gunpowder, which burn 

 with a gradual building up to the final state, and are used, as the name 

 implies, to drive a shell, rocket, airplane, etc. 



From the point of view of phenomena which occur as a result of an 

 explosive set off underwater, the explosion process is of interest chiefly 

 because one must know the physical conditions at the boundary of the 

 explosive and surrounding water to calculate what will happen in the 

 water. A determination of these conditions from measurable properties 

 of the explosive material is therefore necessary, and the ways in which 

 this is done are discussed in Chapter 3. The underlying physical rela- 

 tions necessary to a discussion of detonation in the explosive and propa- 

 gation of disturbances in the water are derived in Chapter 2. 



1.2. Dynamical Properties of Water 



As a result of the explosion process, the initial mass of explosive be- 

 comes a very hot mass of gas at tremendous pressures, and it is evident 

 that these conditions cannot persist without affecting the surrounding 

 medium. If this is water, we must consider what changes occur in a 

 body of water as a result of specified forces or displacements, a problem 

 which is part of the field of physics known as hydrodynamics. If we 

 can restrict ourselves to the concept of water as a homogeneous fluid 

 incapable of supporting shearing stresses we have a medium in which 

 the volume can readjust itself to displacements of its boundaries by 

 flow. In addition, changes in pressure on a definite mass result in com- 

 pression (change in volume) of the mass. 



The fact that water is compressible leads to the conclusions that a 

 pressure applied at a localized region in the liquid will be transmitted 

 as a wave disturbance to other points in the liquid with a velocity which, 

 though large, is finite, and that the wave involves local motion of 

 the water and changes in pressure. If the pressure is small enough, the 

 rate of propagation is practically independent of the magnitude of the 



