In the third of four talks, Professor Sir Hermann Bondi , Chief Scientific Adviser in the Ministry of Defence, observes that for the majority of the British public, defence occupies the same position as the sewers. As long as someone else gets on with managing both, we do not have to think about either. PROFESSOR BONDI argues that it is the duty of every citizen to take an interest in defence, if only because it is the first duty of any government to ensure the security of its citizens.