Letter from Joseph Ernest Petavel, 9 Brighton Grove, Rusholme, Manchester, to Dugald Clerk Arranging to meet Clerk to discuss the design of the 'creeper'. A thought has occurred to him with regard to question 4. The greater the hydrostatic pressure around an explosion, the greater the weapon's range of action would be. A submerged submarine would be less able to survive shock. Any weapon should be exploded under the surface, either for a surfaced or submerged submarine. A firing device based on pressure should be possible. 5 December 1914