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extremely powerful shock waves are formed. If the bubble is filled with gas, it may have a longer lifetime, undergoing successive contractions and expansions as the ultra sonic waves pass, at the same time growing by diffusion, The abrupt formation of cavities by the tearing apart of the liquid and the subsequent abrupt collapse form audible "cracks". When cavitation is intense, the noise can be extremely loud, It Is now known that the cavities are usually Initiated by "weak spots" In the liquid, otten around tiny "nuclei" of dissolved or solid particles, it Is known that the more cavities, the easier it is to create cavitation, with less energy Input. When we look at the "building process" of our embryo hailstone, we find that it includes a lot of opportunities to include "extra nuclei" within that embryo. This is another factor in the vulnerability of the embryo stone. _!.~_~~~A~~_?~V~T~TI~~-":"E ~_~~[)~~...I.J_LTFlASON!q SgUND WA'lE;L_____ _ _" HOW DOES A "HAIL CANNON" PRODUCE AN ULTRASONIC SOUNDWAVE? On first observation, a hail cannon looks like a "P9t belly" stove with a funnel like chimney! The thought that this could be a sophisticated sonic wave generator may be too much for the casual observer to accept, but this is indeed what it is, The Cannon is best described as: A cylindrical blast chamber, open at one end by a tapered neck which allows the escaping blast gasses to pass through a confined exit, which is approximately 1/3 the- diameter and 1/6 the length of the cylinder, to discharge vertically out of a tapered sound directing horn. The blast chamber is organised with a fuelling and ignition mechanism to supply the required blast. This blast propels a charge of sound through the neck which is constructed such a way that through rapid passage of gases produces a particular sound wave. This sound wave is propelled vertically through a tapered horn some 4 times the blast chambers length, with an outlet diameter of equal proportion, The mechanism is fuelled and fired every 6 seconds, creating a repetitive series of soundwaves. The soundwave is of an audible frequency, and is able to be clearly differentiated froin the blast. This sound wave can be heard for as long as 30 seconds, as a "whistle". In certain atmospherical conditions a "ring" presumably of water vapour (although not confirmed as such) can be clearly observed travelling at what has been considered at the speed of sound. This ring is absolutely consistent with the sound of the whistle, i,e. it is only observed with the whistle, and clearly travels at a speed consistent with that whistle. In the event that the Cannon is not producing the whistle, the "ring" is not observed, 6 Copyright <0 1994