[112] About the mouth of the river Gambia in Africa, after the annual inundation of the river, the putrefaction of the mud, mixed with animal and vegetable substances, becomes so great, that the birds manifest their disgust by soaring to an immense height in the air. This is a natural consequence of the levity of putrefactive vapours compared with the common atmosphere. As these vapours, however, are composed of several kinds of gases, it is possible that some may descend, while others ascend; and thus the contagious part, tending to the earth, may violently affect the people who are confined among it, while the birds escape; but there is still wanting some positive evidence that ever the true plague did arise from this cause. We now come to the second mode of prevention, viz. removing these local causes which, in the opinion of some, may produce a plague in any country, and, in that of others, may increase or set in action the contagion previously existing. These causes have been enumerated by the late Dr. Smith,[113] in a Dissertation on the pestilential Diseases which at different times appeared in the Athenian, Carthaginian and Roman armies, in the neighbourhood of Syracuse. They are, 1. The climate and season. 2. The situation of the armies; and, 3. Their condition. The climate of the island of Sicily in general he observes is extremely pleasant at some seasons of the year; in the neighbourhood of Syracuse particularly storms are so infrequent during the former part of the year, that the sun is never obscured for a whole day. Even in the month of January, however, the weather is warm, and as the season advances the heat becomes insupportable. In autumn it is rendered somewhat unpleasant and unhealthy by the frequent rains and chillness of the evenings. But, in particular places, during the hottest season, nothing can exceed its unhealthiness. According to Barichten, “the least stagnant water is sufficient, in the heats of summer, to poison the atmosphere: its effects on the countenances of the poor people who live in its vicinity are evident; and a stranger who travels through the island in this season ought to avoid ever passing a night near them.” De Non says, that “as soon as the sun enters the Lion, this country becomes the house of death: fevers of the most malignant kind seize on the imprudent or unfortunate wretch that spends a night near them (ponds and marshes) and few escape with life when attacked by so virulent a disorder.” [113] Med. Repository, vol. ii, p. 367.