12 THE SEQUENCE OF EVENTS 



merit. The features visible above the surface depend considerably on 

 the initial depth of the charge, being quite spectacular for shallow ex- 

 plosions, but becoming virtually undetectable for great depths. The 

 sequence of events in the shallow case may be quite complicated, but 

 three main phases can be distinguished. These result from the arrival 

 of the primary shock wave at the surface, the approach of the gas sphere 

 to the surface, and the final breakthrough of the gases to the atmos- 

 phere. The motion of the water in a positive pressure wave finds no 

 hindrance to its continuance when the shock wave reaches the surface, 

 as the atmosphere cannot supply appreciable resistance by compression. 

 As a result, a reflected wave of negative pressure is formed at the sur- 

 face with a value such that the sum of the direct and reflected pressures 

 is practically zero. The water in the surface layer is thrown up with a 

 velocity proportional to the pressure existing in the direct wave, and a 

 rounded dome of whitish water forms directly above the charge in the 

 next fraction of a second. Regions of the surface further from the 

 charge are disturbed much less, but a rapidly advancing ring of ap- 

 parently darkened water, the "slick," can often be seen spreading out 

 from the charge for hundreds of feet (for, say, a 300 pound charge) , the 

 extent of the region indicating the points to which the shock wave has 

 advanced. If the bubble is sufficiently close to the surface in an early 

 phase of its motion, it may also disturb the surface before its final break- 

 through. The breakthrough occurs at a later time, determined by the 

 depth, and shoots up plumes of spray which may rise hundreds of feet. 

 All these effects become less pronounced for greater initial depths of 

 water. The slick may be seen for a 300 pound charge 400 or 500 feet 

 deep, but the dome is insignificant and no trace of plumes is evident even 

 at much smaller depths. 



As already mentioned, the pressure field in the water is usually com- 

 plicated by reflections from the surface and bottom, the reflected wave 

 from a free surface being negative and from a rigid surface positive. 

 The resultant pressures observed are superpositions of the direct and 

 reflected waves. A complication arises for reflected waves of negative 

 pressure because the maximum absolute tension sea water can with- 

 stand is certainly quite small and is very probably less than an at- 

 mosphere, if not zero. As a result, while a reflected negative wave can 

 reduce the resultant absolute pressure at any point to zero, appreciable 

 negative pressures do not exist under any ordinary conditions. The 

 reflected wave from the bottom is usually quite irregular and much 

 smaller than would be expected for perfect reflection. In the extreme 

 case of a charge fired on the bottom, the increase in pressure is of the 

 order ten to fifteen per cent, roughly half what would be ideally real- 

 ized. (Although it might seem that the pressure should be doubled in 

 the ideal case, the increase is much less because the energy transport, 



