Abstract: A method and apparatus adapted to provide a time delay between the hydraulic actuation of the firing head assembly by fluid pressure and the detonation of a downhole explosive device used in subterranean wells, such as a perforating gun. In a preferred embodiment, the firing head assembly can be quickly and easily connected in a tubing string intermediate the ported sub and the perforating gun or other explosive device. In one preferred embodiment, the hydraulically-actuated firing head assembly is provided with five combustible columns which are generally coextensive. When the fluid pressure applied to the firing head assembly is sufficient to actuate the firing head assembly, the actuation mechanism, such as a firing piston, initiates a combustive reaction in one end of the first combustible column. When the combustive reaction reaches the other end of the first combustible column, it is communicated to one end of the second combustible column.