I must confess I have not hitherto been able to find any satisfactory explanation of the phenomena which take place during Night-Mare. The following observations which I have been able to make on the paroxysm itself, and on the more immediate causes which give rise to it, may possibly direct some happier genius to a more correct idea of its nature. 1st. I have frequently experienced that this affection may occur in very profound sleep, without breaking or disturbing that sleep, any farther than to produce a dream, the memory of which will remain after waking, in the same manner as that of any other impressive dream. Thus I have frequently dreamed that I had Night-Mare; supposing myself to be in a place, and under circumstances very different to those in which I really was. Under these circumstances I have never become conscious of the real situation in which I was, but on waking, remembered very well that I had been dreaming of labouring under Night-Mare. From this circumstance I conclude, that the Night-Mare is not, as Dr. Darwin insinuates, a state of imperfect sleep, but an absolute disease, which may occur, and go through its regular stages, whilst the patient continues to be, to all intents and purposes, in a natural sleep. There is no doubt but the disease, in this case, is much slighter than ordinary, although the impression made upon the mind may be very strong. 2nd. It happens still more frequently, that the patient is in a state apparently between sleeping and waking, when the paroxysm takes place; which has given rise to the belief, in many persons, that it came on while awake. I have often heard it described by others, (and have felt it myself) as a sense of weight, first upon the feet, and progressively advancing towards the breast, until the paroxysm becomes complete. In the beginning of this state of things, it appears to the patient that he might easily, if he would, move himself, and shake off the incumbent weight. He does not feel at all aware at that moment, of the suspension of the power of volition; nor does he feel any inconvenience from that circumstance until he begins to experience a necessity of exerting that power, in order to carry on the function of respiration. It is not until the involuntary action of the muscles concerned in respiration, in some measure ceases, that the patient perceives that he is absolutely labouring under Night-Mare.