Page:Notes on the State of Virginia (1853).djvu/101

Rh and a moderate current of wind. They are most frequent about sunset; rare in the middle parts of the day; and I do not recollect having ever met with them in the morning.

The variation in the weight of our atmosphere, as indicated by the barometer, is not equal to two inches of mercury. During twelve months observation at Williamsburgh, the extremes were 29, and 30.86 inches, the difference being 1.86 of an inch; and in nine months, during which the height of the mercury was noted at Monticello, the extremes were 28.48 and 29.69 inches, the variation being 1.21 of an inch. A gentleman, who has observed his barometer many years, assures me it has never varied two inches. Cotemporary observations, made at Monticello and Williamsburgh, proved the variations in the weight of air to be simultaneous and corresponding in these two places.

Our changes from heat to cold, and cold to heat, are very sudden and great. The mercury in Fahrenheit's thermometer has been known to descend from 92° to 47° in thirteen hours; and in a single and most remarkable instance, on the 4th of July, 1793, in Orange county, it fell from 84° to 74° in ten minutes.

It is taken for granted, that the preceding table of averaged heat will not give a false idea on this subject, as it proposes to state only the ordinary heat and cold of each month, and not those which are extraordinary. At Williamsburgh in August, 1766, the mercury in Fahrenheit's thermometer was at 98°, corresponding with 29⅓ of Reaumur. At the same place in January, 1780, it was at 6°, corresponding with 11½ below 0 of Reaumur. I believe these may be considered to be nearly the extremes of heat and cold in that part of the country. The latter may most certainly, as at that time York river, at York town, was frozen over, so that people walked across it; a circumstance which proves it to have been colder than the Winter of 1740, 1741, usually called the cold Winter, when York river