V. Sudden changes from great heat to cold, or cool weather, if they occur before the 20th of August, seldom produce fevers. After that time, they are generally followed by them. VI. The same state of the atmosphere, whether cold or warm, moist or dry, continued for a long time, without any material changes, is always healthy. Acute and inflammatory fevers were in vain looked for in the cold winter of 1779-80. The dry summer of 1782, and the wet summer of 1788, were likewise uncommonly healthy in the city of Philadelphia. These facts extend only to those diseases which depend upon the sensible qualities of the air, for diseases from miasmata and contagion, are less influenced by the uniformity of the weather. The autumn of 1780 was very sickly in Philadelphia, from the peculiar situation of the grounds in the neighbourhood of the city, while the country was uncommonly healthy. The dry summer and autumn of 1782 were uncommonly sickly in the country, from the extensive sources of morbid exhalations which were left by the diminution of the waters in the creeks and rivers. VII. Diseases are often _generated_ in one season and _produced_ in another. Hence we frequently observe fevers of different kinds to _follow_ every species of the weather that was mentioned in the last observation. VIII. The excessive heat in Pennsylvania has sometimes proved fatal to persons who have been much exposed to it. Its morbid effects discover themselves by a difficulty of breathing, a general languor, and, in some instances, by a numbness and an immobility of the extremities. The excessive cold in Pennsylvania has more frequently proved fatal, but it has been chiefly to those persons who have sought a defence from it, by large draughts of spiritous liquors. Its operation in bringing on sleepiness previous to death, is well known. On the 5th of February, 1788, many people were affected by the cold. It produced a violent pain in the head; and, in one instance, a sickness at the stomach, and a vomiting appeared to be the consequence of it. I have frequently observed that a greater number of old people die, during the continuance of extreme cold and warm weather, than in the same number of days in moderate weather. IX. May and June are usually the healthiest months in the year. X. The influence of the winds upon health, depends very much upon the nature of the country over which they pass. Winds which pass over mill-dams and marshes in August and September, generally carry with them the seeds of fevers. XI. The country in the neighbourhood of Philadelphia was formerly more sickly than the central parts of the city, after the 20th of August. Since the year 1793, the reverse of this has been the case. XII. The night-air is always unwholesome from the 20th of August, especially during the passive state of the system in _sleep_. The frequent and sudden changes of the air from heat to cold render it unsafe to sleep with open windows, during the autumnal months.