II. I come now, according to the order I proposed, to mention the means of preserving whole cities or communities from the influence of those morbid exhalations which produce the different forms of summer and autumnal disease, and, in particular, that which is of a malignant nature. As the flight of a whole city is rarely practicable, it will be necessary to point out the means of destroying the morbid miasmata. 1. Where the putrid matters which emit them are of a small extent, they should be covered with water or earth. Purchas tells us, 500 persons less died of the plague the day after the Nile overflowed the grounds which had emitted the putrid exhalations that produced it, than had died the day before. During the prevalence of a malignant fever, it will be unsafe to remove putrid matters. A plague was generated by an attempt to remove the filth which had accumulated on the banks of the waters which surround the city of Mantua, during the summer and autumnal months[18]. Even a shower of rain, by disturbing the green pellicle which is sometimes formed over putrid matters, I shall mention in another place, has let loose exhalations that have produced a pestilential disease. [18] Burserus. 2. Impregnating the air with certain effluvia, which act either by destroying miasmata by means of mixture, or by exciting a new action in the system, has, in some instances, checked the progress of a malignant fever. The air extricated from fermenting wines, during a plentiful vintage, Vansweiten tells us, has once checked the ravages of a plague in Germany. Ambrose Parey informs us, the plague was checked in a city in Italy by killing all the cats and dogs in the place, and leaving them to putrify in the streets. Mr. Bruce relates, that all those persons who lived in smoky houses, in one of the countries which he visited, escaped bilious fevers, and Dr. Clark mentions an instance, in which several cooks, who were constantly exposed to smoke, escaped a fever which affected the whole crew of a galley. The yellow fever has never appeared within the limits of the effluvia of the sal ammoniac manufactory, nor of the tan-pits in the suburbs of Philadelphia, nor has the city of London been visited with a plague since its inhabitants have used sea-coal for fuel. But other causes have contributed more certainly to the exemption of that city from the plague for upwards of a century, one of which shall be mentioned under our next head.