In the articles above referred to, facts were brought together which had been collected from the writings of men who had devoted years to the study of pestilences in Spain, in various ports of the Mediterranean, in Constantinople, and in the West Indies. They had gone where epidemics were raging, had risked their lives that they might increase the store of knowledge about these fearful scourges, and might, if possible, learn on what they depend. Amongst these men, one of the most distinguished was apparently a Dr Maclean, of whom the article tells us that "when he was in Spain in 1821 yellow fever attacked Barcelona, and that with his wonted zeal he hastened to the spot in order that he might fully investigate its nature." Dr Maclean is spoken of as "one of those extraordinary men who is capable of concentrating all the faculties of his mind, and of devoting the best years of his life, to the accomplishment of one great and benevolent object." We are told how, "in order to demonstrate what epidemic diseases really are, and what they are not, and to put an end to errors which have so long and so universally prevailed on this subject, errors which he believes to be the source of incalculable misery and of certain death to millions of the human race, Dr Maclean, with an energy scarcely to be paralleled, has devoted thirty years--a large portion of the active life of man. In this cause he has repeatedly risked that life, and for its sake he has encountered all sorts of suspicion and abuse."[5] Generalising, then, from the facts which such men had collected and from others observed by himself, Dr Southwood Smith endeavours to establish the laws of epidemic disease. In the first place, he labours to prove that _epidemic_ diseases are not, in the strict sense of the word, _contagious_, and that the laws which epidemic diseases observe offer a complete contrast to those which regulate contagious diseases. "It was proved," he thought, to use his own words, "that the symptoms of epidemic diseases are not determinate and uniform. They vary in different countries and different seasons--even in the same country and the same season, and do not succeed each other in any determinate order. "That epidemics observe certain seasons--the periods at which they commence, decline, and cease, hardly vary. For instance, the plague in Egypt begins in March or April, and ends in June or July. All epidemics in Great Britain, of which we have any record, have raged in the autumn.