Note 112 — PART FIRST. (continued 7)
Reading edition
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.