This intense cold was not confined to Pennsylvania. In Norfolk, in Virginia, the mercury stood at 18° above 0 on the 22d of January. At Lexington, in Kentucky, it stood at 0 on the 21st of the same month. In Lower Canada the snow was seven feet in depth, which is three feet deeper than in common years. And such was the quantity of ice collected in the northern seas, that a ship was destroyed, and several vessels injured, by large masses of it, floating between the 41st and 42d degrees of north latitude.
Great fears were entertained of an inundation in Pennsylvania, from a sudden thaw of the immense quantities of snow and ice that had accumulated during the winter, in every part of the state; but happily they both dissolved away so gradually, as scarcely to injure a bridge or a road. On the 28th of February the Delaware was navigable, and on the 2d of March no ice was to be seen in it.
Having premised these general remarks, I proceed to observe, that there are seldom more than twenty or thirty days in summer or winter, in Pennsylvania, in which the mercury rises above 80° in the former, or falls below 30° in the latter season. Some old people have remarked, that the number of extremely cold and warm days in successive summers and winters, bears an exact proportion to each other. This was strictly true in the years 1787 and 1788.
The warmest part of the day in summer is at two, in ordinary, and at three o'clock in the afternoon, in extremely warm weather. From these hours, the heat gradually diminishes till the ensuing morning. The coolest part of the four and twenty hours, is at the break of day. There are seldom more than three or four nights in a summer in which the heat of the air is nearly the same as in the preceding day. After the warmest days, the evenings are generally agreeable, and often delightful. The higher the mercury rises in the day time, the lower it falls the succeeding night. The mercury at 80° generally falls to 68°, while it descends, when at 60°, but to 56°. This disproportion between the temperature of the day and night, in summer is always greatest in the month of August. The dews at this time are heavy in proportion to the coolness of the evening. They are sometimes so considerable as to wet the clothes; and there are instances in which marsh-meadows, and even creeks, which have been dry during the summer, have been supplied with their usual waters from no other source, than the dews which have fallen in this month, or in the first weeks of September.
There is another circumstance connected with the one just mentioned, which contributes very much to mitigate the heat of summer, and that is, it seldom continues more than two or three days without being succeeded with showers of rain, accompanied sometimes by thunder and lightning, and afterwards by a north-west wind, which produces a coolness in the air that is highly invigorating and agreeable.
The warmest weather is generally in the month of July. But intensely warm days are often felt in May, June, August, and September. In the annexed table of the weather for the year 1787, there is an exception to the first of these remarks. It shows that the mean heat of August was greater by a few degrees than that of July.
The transitions from heat to cold are often very sudden, and sometimes to very distant degrees. After a day in which the mercury has stood at 86° and even 90°, it sometimes falls, in the course of a single night, to the 65th, and even to the 60th degree, insomuch that fires have been found necessary the ensuing morning, especially if the change in the temperature of the air has been accompanied by rain and a south-east wind. In a summer month, in the year 1775, the mercury was observed to fall 20° in an hour and a half. There are few summers in which fires are not agreeable during some parts of them. My ingenious friend, Mr. David Rittenhouse, whose talent for accurate observation extends alike to all subjects, informed me, that he had never passed a summer, during his residence in the country, without discovering frost in every month of the year, except July.