166 
III. PRESSURE WAVE 6 
1. OBSERVATIONS 
1. OBSERVATIONS ON THE PRESSURE WAVE 
ABBOT'S OBSERVATIONS (1869-1881), (4), the first extensive investigation, 
are chiefly of historical interest, because the interpretation of his data is open 
to question. The following conclusions from his work may be cited. 
(a) Gunpowder gave erratic results, but the results produced by dynamite 
or guncotton were very consistent. 
(b) The "mean" pressure in the water was found by Abbot to vary in proportion 
to the linear dimensions of the charge, and as 1/r‘* where r is the distance of the 
point of observation from the center of the explosion (more recent work indicates a 
variation as 1/r). 
The following remark may also be quoted from his report as constituting 
early evidence pointing toward a multiplicity of some sort in the pressure wave: "It 
is a general characteristic of small and deeply submerged charges of the explosive 
compounds, and of some quick-acting explosive mixtures as well, that at the instant 
of detonation, before any disturbance of the water at the surface is visible, three 
sharp sounds are heard... . of nearly equal intensity," the interval of time be- 
tweea the last two being shorter than that between the first two. He states also 
that successive impulses are felt by a person standing in a boat. 
HILLIAR'S OBSERVATIONS, (1), published by the English Department of Scien- 
tific Research and Experiment in 1919, are the best so far available. The report in- 
cludes observations of the pressure wave, of the surface effects, and of relative 
damage to targets. 
The pressure wave was studied chiefly by means of what might be called 
"impulse crusher gauges." The working part was a steel piston several inches long 
and half an inch in diameter, set in motion by the water pressing on the outer end. 
After traveling a known distance, the piston struck a short cylinder of copper. From 
the shortening produced in the copper the final velocity and momentum of the piston 
were calculated. The momentum was taken as a measure of the impulse {pdt in the wave 
from the start up to the instant at which the piston struck the copper. The coppers 
were calibrated by striking them with pistons moving at measured velocities. The 
shortening was found to be proportional to the energy of the blow, regardless of the 
weight of the piston. By using several gauges with pistons having various distances 
of free travel, all mounted at the same distance from the explosion, various portions 
of the total impulse could be measured; and from the calculated values of the final 
velocities of the pistons and their distances of travel, the times could be calculated. 
Maximum pressures were also measured with a crusher gauge in which a plate, 
actuated by the water, crushed a copper with which it was initially in contact. 
There is a possible source of error in the use of such gauges which is not 
discussed by Hilliar. Since the pistons moved parallel to the wave front (vertically), 
they would be caused to press against the wall of the hole owing to acceleration of 
