It may seem strange that dreams of flying, if so often due to organic disturbances, should usually be agreeable in character. It is not, however, necessary to assume that they are caused by serious interference with physiological functions; often, indeed, they may simply be due to the presence of a stage of consciousness in which respiration has become unduly prominent, as it is apt to be in the early stage of nitrous oxide anaesthesia, that is to say, to a relative wakefulness of the respiratory centres. It would seem that the disturbance is frequently almost, or quite, imperceptible on waking, and by no means to be compared with the more acute organic disturbances which result in dreams of murder, although it may be of nervous origin.[123] In some cases, however, it appears that dreams of flying are accompanied by circumstances of terror. Thus a medical correspondent, who describes his health as fairly good, writes in regard to dreams of flying: 'I have often had such dreams, and have wondered if others have them. Mine, however, are not so much dreams of flying, as dreams of being entirely devoid of weight, and of rising and falling at will. A singular feature of these levitation dreams is that they are always accompanied by an intense and agonising fear of an evil presence, a presence that I do not see but seem to feel, and my greatest terror is that I _shall_ see it. The presence is ill-defined, but very real, and it seems to suggest the potentiality of all possible moral, mental, and physical evil. In these dreams it always occurs to me that if this evil presence shall ever become embodied into a something that I could _see_, the sight of it would be so ineffably horrible as to drive me mad. So vivid has this fear been that on several occasions I have awakened in a cold sweat or a nameless fear that would persist for some minutes after I realised that I had only been dreaming.' This seems to be an abnormal type of the dream of flight. It is somewhat surprising that while dreams of floating in the air are so common and clearly indicate the respiratory source of the dream, dreams of floating on water seem to be rare, for as the actual experience of floating on water is fairly familiar, we might have expected that sleeping consciousness would have found here rather than in the never experienced idea of floating in air the explanation of its sensations. The dream of floating on water is, however, by no means unknown; thus Rachilde (Mme. Vallette), the French novelist and critic, whose dream life is vivid and remarkable, states that her most agreeable dream is that of floating on the surface of warm and transparent lakes or rivers.[124] One of the correspondents of _L'intermédiaire des Chercheurs et des Curieux_[125] also states that he has often dreamed of walking on the water.