On Storms and Meteorological Observations. 369 



through it, that when collected in the focus of a burning glass they 

 would scarce kindle brown paper. Of course their summer effect 

 in heating the earth was exceedingly diminished. Hence the sur- 

 face was easily frozen, and the first snows remained on it unmelted 

 and, received continual additions. Hence perhaps the winter of 

 1783-4 was more severe than any that had happened for many 

 years. The cause of this universal fog is not yet ascertained."* 



"In my descent from Pittsburg to Louisville, I found the wind, , 

 excepting about two days, constantly blowing up the river. The 

 north west or south west winds in fact continue almost three quarters 

 of the year. The deep valley which the river has excavated forms 

 a vortex, into which the rarefied air of the land rushes for equilibri- 

 um. The south west wind is uniformly, at this season of the year, 

 (latter part of November,) attended with a dense and bluish atmos- 

 phere, charged with vapors, which appear like smoke and sometimes 

 accumulate so as to obscure the land." 



"Among the more remarkable features of the autumnal season in 

 this country, (Arkansaw,) is the aspect of the atmosphere, which in 

 all directions appears so filled with smoke as often to render an object 

 obscure at the distance of one hundred yards. The south west winds 

 at this season are often remarkably hazy, but here the effect is great- 

 ly augmented by the burning of the surrounding prairies."f 



See Dobson on the Harmattan, in the Philosophical Transactions 

 for 1781. — "An extremely dry wind in Africa, coming from the north 

 east, drying even potash. It generally brings a fog of some unknown 

 nature "li. Humboldt promised a discussion of the subject of dry fogs 

 in the Personal Narrative, but it is to be expected of a book a quarter 

 of a century in publication that some things promised in it will be for- 

 gotten and omitted. 



It is very desirable to have the atmospherical changes on the east- 

 ern and western sides of the Alleghanies connected and compared, by 

 means of a series of good observations; but in the case of the winds, 

 the fact noticed by Mr. Nuttall must be attended to — that the winds 

 along the beds of large streams, or in the neighborhood of the sea, 

 determine nothing precisely in regard to the direction of the main 

 currents of the great aerial ocean. 



University of North Carolina, June 7, 1831. 



* Franklin's Works, Vol. Ill, p. 288. 

 t Nuttall's Travels, pages 35 and 217^ 

 t Young's Philosophy, Vol. II, p. 458. 



