9 (5) (r,2) = - de. (3) (8 - 2)2+ 12 It should be emphasized that the free surface is assumed to be located at z = 0, approximately at the same place where it was at the moment of the bullet impact, which is a good approximation for sufficiently slender bullets. The strength of the sources distributed over the submerged and mirror-reflected axis of the bullet is established using the fact that the bullet surface is impenetrable for blood and thus . 9(E) a (4 ar bullet surface 271 / / bullet surface