Page:Notes on the State of Virginia (1802).djvu/120

110 to volcanos, becauſe we have none. They do not happen in the winter when the farmers kindle large fires in clearing up their grounds. They are not confined to the ſpring ſeaſon, when we have fires which traverſe whole countries, conſuming the leaves which have fallen from the trees. And they are too frequent and general to be aſcribed to accidental fires. I am perſuaded their cauſe muſt be ſought for in the atmoſphere itſelf, to aid us in which I know but of theſe conſtant circumſtances; a dry air; a temperature as warm at leaſt as to that of the ſpring or autumn; and a moderate current of wind. They are moſt frequent about ſun-ſet: 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 Williamſburg, 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 Farenheit's thermometer his been known to deſcend from 92° to 47° in thirteen hours.

It is taken for granted, that the preceding table of average heat will not give a falſe idea on this