Aerial defences against Zeppelins were haphazard, and divided between the RNAS and the Royal Flying Corps (RFC), with the Navy engaging enemy airships approaching the coast while the RFC took responsibility once the enemy had crossed the coastline. Initially, the War Office believed that the Zeppelins used a layer of inert gas to protect themselves from incendiary bullets, and discouraged the use of such ammunition in favour of bombs. The initial trials of incendiary bullets in mid-1915 were unimpressive, and the explosive Pomeroy bullet attracted little official interest. However experiments undertaken in 1916 using a mixture of explosive and incendiary rounds were promising, and the use of this mixture of Pomeroy, Brock, and Buckingham bullets brought the defending aircraft their first victories, using the technique of firing machine guns upwards at 45 degrees into the airship.