The Royal Air Service is a branch of the British military. The Air Service has naval ranks and seems to be composed of Darwinist airships and their crew and supporting creatures. Two ships in the Air Service are the Leviathan and the Minotaur.
The Royal Air Service seems to be the equivalent of the Royal Naval Air Service in the real world. Unlike the R.N.A.S. in the real, world, though, the Royal Air Service is the main part of the British air forces. In the real world, the Royal Flying Corps, which was part of the army, was the primary air force of the British military.
The Air Service is under the authority of the Admiralty, and is probably part of the navy.