classical analogy, of course, is that of the Sibyls, who delivered the oracular responses from the tripod, under the afflatus of a vapour said to arise from the sacred subterranean stream called Kassotis. The theory is, therefore, by no means a new one!”

Major Dale stared dully, but made no attempt to interrupt me.

“There are probably many spots, in England alone,” I continued, “thus affected; but, fortunately, few of them have been chosen as building-sites. Barrows and tumuli of the stone and bronze age, and also Roman shrines, seem frequently to be productive of such emanations. The barrow beside Low Fennel (and now under the new wing) is a case in point.

“Sudden atmospheric changes seem to be favourable to the formation of the vapour. The barrow in Peel Castle, Isle of Man, is peculiarly susceptible to thunder-storms, for instance, whilst[58] that at Low Fennel emits a vapour only after a spell of intense heat, and at the exact moment when the temperature begins to fall again. In the case of a sustained heat-wave, this would take place at some time during each night.

[58]

“And now for the particular in which the vapour at Low Fennel differs from other, similar emanations. It is not productive of delusions of sight; it induces a definite and unvarying form of transient insanity!”

Major Dale moved slightly, but still did not speak.

“Dame Pryce was the first recorded victim of the vapour. She was accused of witchcraft by a neighbour who testified to having seen her transform herself into a hideous and unrecognizable hag—whereas, in her proper person, she seems to have been a comely old lady. Lack of evidence compels us to dismiss the case of Seager, but consider that of the Ords. The man Ord, on his own confession, had fallen asleep outside the house. He became a victim of the vapour—and his own wife failed to recognize him.

“To what extent the mania so produced is homicidal remains to be proved; the gas is rare and difficult to procure, so that hitherto analysis[59] has not been attempted. My own theory is that the subject remains harmless provided that, whilst under the mysterious influence, he does not encounter any person distasteful to him. Thus, Seager may have met his death at the hands of some tramp who had been turned away from the house.

[59]

“As to the symptoms: 

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