256 WRITINGS OF JOSEPH HENRY. [1855- 



amined microscopically by Professor G. C. Schaeffer of Wash- 

 ington, at the request of the Smithsonian Institution, have 

 been found to consist of portions of plants which must have 

 come from a great distance, since the species to which they 

 belong are not found in abundance in the localities at which 

 the specimens were obtained. It is highly probable that a 

 portion of the smoke or fog-cloud produced by the burning 

 of one of our western prairies is carried entirely across the 

 eastern portion of the continent to the ocean. On this sub- 

 ject Dr. Smallwood communicated a series of interesting 

 observations to the American Association at their meeting 

 in Albany in 1855. Particles of matter of the kind we have 

 described are good absorbers and radiators of heat, and hence 

 in the daytime they must become warmer than the surround- 

 ing atmosphere and tend to be buoyed up by the expansion 

 of the air which exists in the interstices between them, while 

 at night they become cooler by radiation than the surround- 

 ing air and tend to condense upon themselves the neigh- 

 boring moisture, and consequently to sink to a lower level. 

 It is on this account that the smoky clouds which are pro- 

 duced by the enterprising manufacturing establishments of 

 Pittsburg and other western cities, sometimes descend in still 

 weather to the surface of the earth and envelop the inhabi- 

 tants in a sable curtain more indicative of material prosper- 

 ity than of domestic comfort. From the density and the 

 wide diffusion of these smoky clouds they must produce a 

 sensible effect upon the temperature of the season of the year 

 in which they occur. During a still night, when a cloud of 

 this kind is over head, no dew is produced; the heat which 

 is radiated from the earth is reflected, or absorbed and radi- 

 ated back again, by the particles of soot, and thus the cooling 

 of the earth necessary to produce the deposition of water in 

 the form of dew and hoar frost is prevented. 



So well aware of this fact are the inhabitants of some parts 

 of Switzerland that, according to a paper by Boussingault, in 

 a late number of the Annates de Chimie, they kindle large 

 fires in the vicinity of their vine fields and cover them with 

 brush to produce a smoke-cloud by which to defend the 



