1. Rifle bullets ordinarily have muzzle velocities around 2000–3000 ft.∕sec., though with special devices projectiles have been shot from gun barrels up to 5000 ft.∕sec. F. Zwicky and F. Whipple have suggested using these jets as artificial meteors for controlled studies, since their velocities are about the same as the velocities of the slower meteors. Shaped charges are carried to the upper atmosphere by V-2 rockets and exploded when the rocket has reached its highest point. Calculations indicate that these “meteors” should be observable from the ground.
2. Like most analogies this is far from perfect, for while a stream of water washes mudoutof a mud bank the jet of metal does not wash or erode metal out of the target. Careful weighings have shown that a metal jet is captured by a metal target, which loses no weight except a very small amount at the front surface. The hole is produced by plastic flow of the target material in a radial direction.
3. Robert H. McLemore, “Formation penetrating with shaped explosive charges,” Oil Weekly July 8 (1946).
4. P. W. Bridgman has measured static pressures up to 100, 000 atmospheres, in spaces 516 in. long and 116 in. thick, and has obtained pressures estimated at 400, 000 atmospheres but was unable to make observations at the higher pressures.
5. Much pioneer work toward understanding this phenomenon was done by W. M. Evans and A. R. Ubbelohde under the auspices of the British Ministry of Supply.