"Ah?" said the Professor, with the bland superiority of his tribe. "Well, well! Yet even you know something of the evils attending people who live in low, swampy areas; malaria, ague, fevers. In the tropics, these take the form of virulent maladies that sweep a man from earth in a few hours. Your lake _was_ haunted, so was the house that once stood in its basin, as some vague instinct strove to warn the generations of Michells as well as you. Haunted by emanations of some powerful form of marsh gas given forth more plentifully at night, which lowered the heart action and impeded the breathing of one drawing the poison into his lungs through hours of sleep, producing--nightmare. Science has by no means analyzed all the possibilities of such phenomena." "Nightmare!" I cried. "Do you mean to account by nightmare for the wide and repeated experiences that twice brought me to the verge of death? And Desire? What of her knowledge of that same nightmare? What of the legend of her family so exactly coinciding with all I felt? And why did not Phillida and Ethan suffer the nightmare with me?" He held up a lean hand. "Gently, gently, Roger! Consider that of all the household you alone slept in the side of the house toward the lake. I know that you always have your windows open day and night--a habit that used to cause great annoyance to your Aunt Caroline when you were a boy. Thus you were exposed to the full effect of the water gases. That you did not feel the effects every night I attribute to differences in the wind, that from some directions would blow the fumes away from the house, thus relieving you. I gather from your account that the phenomena were most pronounced in close, foggy weather, when the poisonous air was atmospherically held down to the earth. You have spoken of miasmic mists that hung below the level of the tree-tops. When Mr. Vere experienced a similar unease and depression, he was on the shore of the lake at dawn after precisely such a close, foggy night as I have described as most dangerous. The symptoms confirm this theory. You say you awakened on each occasion with a sense of suffocation. Your heart labored, your limbs were cold and mind unnaturally depressed, owing to slow circulation of the blood. You were a man asphyxiated. After each attack you were more sensitive to the next, as a malaria patient grows worse if he remains in the swamp districts. It is remarkable that you did not guess the truth from the smell of decaying vegetation and stagnant damp which you admit accompanied the seizures! However, you did not; and in your condition the last three days of continuous fog brought on two attacks that nearly proved fatal. Now as to the character of your hallucinations, and their agreement with the young lady's ideas. That is a trifle more involved discussion, yet simple, simple!" He put the tips of his fingers together and surveyed us with the benign condescension of one instructing a class of small children.