A curious instance of atmospheric pollution came to my notice on November 26 while walking from Hayfield into Edale over the central watershed of England in the Peak of Derbyshire. Below 1500 ft. on the western side there was hazy sunshine with a rime frost, but at this level we entered a thin cloud, with a temperature scarcely below freezing point, formed by a steady easterly wind blowing over the Peak plateau from the east. Friends in Edale informed us that these conditions had obtained since the morning of the previous day, and the hoar-frost thus formed was peculiar; the stream-lines of air-flow round stones were clearly mapped by curling lines of ice, while every blade of grass and stem of Juncus bore a deposit of ice-crystals which resembled the blade of a knife, the knife-edge pointing up-wind and the parent leaf forming the back.