82 



Going out into the open air, in the temperate, and 

 warm months of the year, we often meet with hodies 

 of warm air, which passing hy iis in two or three se- 

 conds, do not aiford time to tlie most sensible ther- 

 mometer to seize their temperature. Judging from my 

 feelings only, I think they approach the ordinary heat 

 of the human body. Some of then) perhaps go a little 

 beyond it. They are of about '20 or 30 feet diameter 

 horizontally. Of their heighth we have no experience, 

 but probably they are globular volumes wafted or rolled 

 along with the wind. But whence taken, where found, 

 or how generated ? They are not to be ascribed to vol- 

 canoes, because we have none. They do not happen in 

 the winter when the farmers kindle large fires in clear- 

 ing up their grounds. They are not confined to the 

 spring season, when we have fires which traverse whole 

 counties, 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 circum- 

 stances ; a dry air ; a temperature as warm at least as 

 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 in- 

 dicated by the barometer, is not equal to two inches of 

 mercury. During twelve months observation at Wil- 

 liamsburgh, the extremes were 29, and 30.86 inches, 

 the difference being 1.86 of an inch: and in nine 

 months, during which the heighth 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 gentle- 

 man, who has observed his barometer many years, as- 

 sures me it has never varied two inches. Contempora- 

 ry observations, made at Monticello and Williamsburgh, 

 proved the variations in the weight of air to be simul- 

 taneous and corresponding in tliese two places. 



Our changes from heat to cold, and cold to heat, are 



