Besides climate and season, the other circumstances determining to the one disease more than the other are, 1. A difference in the constitutions of different men; for in the same ship it sometimes happens that both diseases prevail equally, though all the men are using the same diet and breathing the same air. 2. The nature of the occasional cause. A dysentery, for instance, is more likely to arise from an irregularity in eating or drinking; a fever from being exposed to the weather, particularly marsh effluvia. 3. The particular species of infection that may happen to be introduced. Suppose, for example, that a ship’s company is predisposed to acute distempers, and one man or more ill of the dysentery should be brought on board, this will become the prevailing disease, as happened in the Torbay in August, 1780. If the like number of fevers should be introduced, then fevers will be the prevailing disease. These two diseases may therefore be considered as _vicarious_, the one substituting itself for the other according to particular accidents, and both proceeding from the same general causes; and this is no new idea of mine, but seems to have been Dr. Sydenham’s, when he calls the dysentery a _febris introversa_. It may be farther added, that dysentery is the latest form in which this cause, which is common to both, can exert itself; for it is a disease more within the reach of art; and some of the most dangerous symptoms attending fevers, particularly _delirium_, seldom occur in dysentery. When it proves fatal, it is in consequence of violent local affection, and that in general after it has taken a chronic form. When an incipient fever turns into a dysentery, all the symptoms, and particularly the head-ach, delirium, and _coma_, if there should be any, are immediately relieved. And the most favourable cases of the yellow fever are those in which a bilious diarrhœa comes on, while the most fatal are those in which the bowels are so torpid as to be insensible to any stimulus either from their own contents or from medicine. I shall not enter into a minute description of this disease in all its stages, as this has been so ably executed by Sir John Pringle, Sir George Baker, and other authors, but shall only give a sketch of some of the most remarkable symptoms, particularly such as are peculiar to the climate and manner of life, so as to explain the varieties that may be necessary in the mode of treatment.