540 - 2 - 



The depth of charge and target plate centre was kept throughout the series at five feet as 

 at this depth the effect of free surface and rigid bottom almost exactly cancel the influence of 

 gravity during the first oscillation. The migration of the bubble up to its first miniciura was thus 

 almost entir.-ly due to the attractive effect of the rigid box model. 



'Measurement of Photographs . 



From the silhouettes of the bubbles, volumes and positions of the centres of gravity of the 

 bubbles were estimated by the method described in reference (u). as the effect of gravity approximately 

 cancelled the free surface and rigid bottom effects the bubble* were assumed to be symmetrical about a 

 horizontal axis normal to and through the centre of the target plate. The actual appearance of the 

 bubblts up to their first minima suggested that this was a reasonable assumption, but iftmediately after 

 the minimum th"; bubbles became considerably distorted and quite c1e?.rly hrd no 3xis of symmetry. 

 The measurement of volumes and positions of centre of gravity of the buDblfS is thus subject to 

 considerable error after the minimum size has been passed. 



zesul t s^ . 



Typical photographs are shown in Figures 2, 3 and u in which the following features may be 

 noted. Before the minimum the bubble is relitively stnooth, apart from thin needle like streamers 

 ?.t the surface^ /.nd is elongated in a direction normal to the target plate. On reaching the minimum 

 size the bubble moves quickly in towards the target plate, becomes very distorted in general shape 

 and shows a vriry irregular uneven surface. A short tim? before the minimum the surface of the 

 bubble farthest from the target plate develops i very marked "plume-like" structure, most clearly 

 seen in Figure 4, which is left behind as the bubble moves in towards the plate. This plume appears 

 to be related to the wake behind the moving bubble. It is not certain whether it consists of a 

 trail of particles of dirt or unbjrnt carbon, or whether it is a stream of fine bubbles. The plume 

 does not oscillate appreciably in size and remains more or less stationary afttr the bubble has spread 

 itself out over the target plate. This same plumt has been observed in other photographs of the 

 bubble produced by i oz. charges in the absence of a target where appreciable rruvoment .f the bubble 

 occurred, ::nd was always situated in the rear of the moving bubble. 



In all shots where the charge was 23 inches or more from the target plate the bubble was 

 self-luminous for a ihort time near its minimum size, and this "flash" produced a bright streak on the 

 moving film, which trey be seen in Figure 2. The production of this flash was noted in reference (3), 

 and is discussed below. 



The quantitative measurements from the photographs are depicted in Figures 5. 6 and 7, and 

 numerical values set out in Tables, I, II and III. Table I contains a summary of the principal 

 quantities determin'd for each shot. 



Radius-Time Curve s, 



In Figure 5 the mean radius of the bubble, defined as the radius of a sphere having the same 

 volume as the bubble, has been plotted against time for a range of values of the initial charge 

 distance. The zero of time is chosen arbitrarily to exhibit the curves in relation to each other 

 to the test advantage. For the larger charge distances the curves are similar to those for a charge 

 in open water away from any target. when the charge was initially 22 inches or less from the target 

 plate the bubble and water acquired a great deal of kinetic energy due to the bubble's motion toward 

 the target; in consequence, the collapse of the bubble was somewhat inhibited, the potential energy 

 in the gas being less in these cases and the corresponding minimum radii greater. Moreover, the 

 subsequent expansion of the bubble also seemed to be less rapid for these closer shots, which may in 

 part have been the result of losing a considerable amount of energy to the target plate, in addition 

 to the probable loss of energy as the result of turbulence. 



The observed values of minimum r=idius are set out in Table II, together with those calculated 

 from equation (7), reference (5) relating the minimum radius to the "momentum constant" m. This 

 momentum constant m is a non-dimensional constant proportional to the linear momentum acquired by the 



bubble 



