This work I have followed up by a treatise on a nervous disorder, which, for the first time, appeared in the same century, as an epidemic, with symptoms that can be accounted for only by the spirit of the Middle Ages—symptoms which, in the manner of the diffusion of the disease among thousands of people, and of its propagation for more than two centuries, exercised a demoniacal influence over the human race, yet in close, though uncongenial alliance, with kindlier feelings. I have prepared materials for various other subjects, so far as the resources at my disposal extend, and I may hope, if circumstances prove favourable, to complete by degrees, the history of a more extensive series of Epidemics on the same plan as the “Black Death,” and the “Dancing Mania.” Amid the accumulated materials which past ages afford, the powers and the life of one individual, even with the aid of previous study, are insufficient to complete a comprehensive history of Epidemics. The zealous activity of many must be exerted if we would speedily possess a work which is so much wanted in order that we may not encounter new epidemics with culpable ignorance of analogous phenomena. How often has it appeared on the breaking out of epidemics, as if the experience of so many centuries had been accumulated in vain. Men gazed at the phenomena with astonishment, and even before they had a just perception of their nature, pronounced their opinions, which, as they were divided into strongly opposed parties, they defended with all the ardour of zealots, wholly unconscious of the majesty of all-governing nature. In the descriptive branches of natural history, a person would infallibly expose himself to the severest censure, who should attempt to describe some hitherto unknown natural production, whether animal or vegetable, if he were ignorant of the allied genera and species, and perhaps neither a botanist nor zoologist; yet an analogous ignorance of epidemics, in those who nevertheless discussed their nature, but too frequently occurred, and men were insensible to the justest reproof. Thus it has ever been, and for this reason we cannot apply to ourselves in this department, the significant words of Bacon, that we are the ancients, and our forefathers the moderns, for we are equally remote, with them, from a scientific and comprehensive knowledge of epidemics. This might, and ought to be otherwise, in an age which, in other respects, may, with justice, boast of a rich diversity of knowledge, and of a rapid progress in the natural sciences.