About one o'clock one Saturday afternoon in summer, a hydro-aeroplane--or, as its owner preferred to call it, a flying-boat--dropped lightly on to the surface of one of the many creeks that intersect the marshes bordering on the river Swale. The pilot, a youth of perhaps twenty years, having moored his vessel to a stake in the bank, leapt ashore with a light suit-case, and walked rapidly along a cinder path towards the low wooden shed, painted black, that broke the level a few hundred yards away.