Pursuing a new air attack technique, Allied bombers in the South-West pacific were increasing their toll of Japanese merchant vessels and warships. The method was known as "skip bombing," in which the aircraft came in on its target at a very low altitude, usually about 50 feet, travelling at high speed and releasing its bombs when close to the objective; the bombs travelled forward horizontally for a distance before hitting the water just in front of the vessel, then bounced up from the surface of the sea against it. The R.A.F was the first to employ this ingenious plan, against land targets in Northern France, in 1941. The Americans adopted it, experimented with it for both land and sea attack, and found it successful against Japanese shipping. In Diagram 1, attacking planes are approaching their objective low above the water, thus presenting difficult targets for enemy fire (A). In Diagram 2 the bomber pilot (B) is aiming his plane at one of the merchant vessels; it was the pilot, judging the crucial moment, not the bomb-aimer (C) who actually released the bombs. When the appropriate striking distance had been reached, the bombs leave the plane, hit the water's surface a glancing blow (D) and then bounce on into the hull or super-structure of the vessel. By this time the plane (E) had passed over the ship and is clear of the danger area. Delayed action bombs were sometimes used; these gave the attacking plane more time to get away from the target before the dropped bombs exploded. Any type of aircraft may skip-bomb. Flying Fortresses have been known to carry out the manoeuvre successfully; but best for the purpose was a medium bomber, such as the Mitchell or Marauder. Bombs used ranged in weights up to 1,000 lb. Beyond that weight, their shape altered considerably and was not adaptable to the bouncing-bomb technique; 250-1b, and 500-lb bombs were mostly used. In misty or cloudy weather, low-level reconnaissance followed by surprise skip-bombing had proved particularly deadly. The main purpose behind the low level method of attack was that the vessels presented a bigger target than they did from a height; it was in fact, a combination of dive-bombing and aerial torpedo attack.