(a)it becomes necessary to stop any vessel or compel any aircraft to land, it shall be lawful for any vessel or aircraft in the service of the Government while flying her proper flag and any authority authorised in this behalf by the Central Government to summon such vessel to stop or the aircraft to land, by means of an international signal, code or other recognized means, and thereupon, such vessel shall forthwith stop or such aircraft shall forthwith land; and if it fails to do so, chase may be given thereto by any vessel or aircraft as aforesaid and if after a gun is fired as a signal the vessel fails to stop or the aircraft fails to land, it may be fired upon;