Dr. Potter states[33] that he witnessed the rise of a most malignant yellow fever, in a valley in Pennsylvania, which contained numerous ponds of fresh water, and which, from the heat and dryness of the season, emitted a most offensive smell: that the fever prevailed most, and with the greatest degree of malignity among the people who lived nearest these ponds; and adds an exceedingly instructive case, illustrative of the generation and operation of this cause of fever, recorded by Major Prior, in his account of a fever which attacked the army of the United States at Galliopolis. The source of the malady was clearly traced to a large pond near the cantonment. When the disease was most severe, it assumed the continued form, and was accompanied with yellowness of the skin: when proper means were taken to destroy the pond, the fever immediately lost its continued form, and became first remittent, then intermittent, and ultimately disappeared. “The fever,” says this intelligent officer, “was, I think, justly charged to a large pond near the cantonment. An attempt had been made two or three years before to fill it up, by felling a number of large trees that grew on and near its margin, and by covering the wood thus fallen with earth. This intention had not been fulfilled. In August, the weather was extremely hot, and uncommonly dry: the water had evaporated considerably, leaving a great quantity of muddy water, with a thick slimy mixture of putrefying vegetables, which emitted a stench almost intolerable. The inhabitants of the village, principally French, and very poor, as well as filthy in their mode of living, began to suffer first, and died so rapidly, that a general consternation seized the whole settlement. The garrison continued healthy for some days, and we began to console ourselves with the hope that we should escape altogether: we were, however, soon undeceived, and the reason of our exemption heretofore was soon discovered. The wind had blown the air arising from the pond from the camp; but, as soon as it shifted to the reverse point, the soldiers began to sicken: in five days, half the garrison was on the sick list, and in ten, half of them were dead. They were generally seized with a chill, followed by headache, pains in the back and limbs, red eyes, constant sickness at stomach, or vomiting, and generally, just before death, with a vomiting of matter like coffee-grounds. They were often yellow before, but almost always after death. The sick died generally on the seventh, ninth, and eleventh days, though sometimes on the fifth, and on the third. As some decisive measures became necessary to save the remainder of the troops, I first thought of changing my quarters, but as the station was in every respect more eligible than any other, and had been made so by much labour and expense, I determined to try the experiment of changing the condition of the pond, from which the disease was believed to have arisen. A ditch