502 



THE SOUTHE 



RN PLANTER 



[AuGusr 



It is, however, frequently observed that 

 fogs are formed during still nights, in low 

 places, when the surface of the ground is 

 colder than the stratum of the atmosphere 

 •whice rests upon it, and, indeed, we have 

 shown that the temperature of the surface 

 of the earth on a still and clear night is al- 

 ways lower than that of the air which is 

 immediately in contact with it; and it is 

 not easy, without further explanation, to see 

 the reason why fogs should not alw^ays be 

 produced in this case as well as dew. When 

 the atmosphere is still, the condensation of 

 the vapour by the coldness of the surface 

 is so gradual that the air is not disturbed, 

 and the strata immediately above the grass 

 has relatively less moisture in it than that 

 a few yards higher ; hence, no fog ought to 

 be produced in this case, since all the pre- 

 cipitation produced is that which has set- 

 tled directly upon the grass in the form of 

 dew. In this ease we may define the dew 

 to be a fog entirely condensed into drops of 

 water. The question still arises, how, un- 

 der these conditions, can a fog really be 

 produced ? The answer is, that another 

 condition is required, namely, that the sur- 

 face, cooled by radiation, should slope to a 

 lower level, as in the side of a hill or the 

 concave surface of the sides of a hollow. 

 In this case the superincumbent stratum of 

 air of v.'hich the temperature has been 

 lowered by contact with the cold earth, 

 flows down the declivity, by its greater 

 weight, into the valley below, and there, 

 mingling with the damp air which gener- 

 ally exists in such places, precipitates a 

 part of its transparent vapour into visible 

 fog. In the way we have described, large 

 hollows are sometimes seen in the morning, 

 filled with a mass of fog, exhibiting a defi- 

 nite and level surface, presenting the ap- 

 pearance of a lake of which the shores are 

 bounded by the surrounding eminences ; 

 and if a depression of sufficient depth oc- 

 curs in any part of the circumference of 

 the basin, through this the fog is seen 

 to flow like a river from the outlet of a 

 lake. 



The explanation we have here given of 

 the formation of fog in low places is also 

 appHcable to the phenomenon, frequently 

 observed, of early frost in the same locali- 

 ties. As rapidly as the air is cooled on the 

 sides of sloping ground it sinks into the 

 valley below, and its place is supplied hy 

 the warmer air above, which has not been 



subjected to the cooling influence. In the 

 vicinity of Washington, the hollows are 

 sometimes found several degrees colder than 

 the more elevated parts of the surrounding 

 surface. Fogs are produced on the ocean 

 when a little wind, charged with moisture, 

 mingles with another of a lower tempera- 

 ture. The wind from the Gulf Stream 

 mixing with the cold air which rests upoa 

 the water from the arctic regions, which, as 

 we have before stated, flows along close to 

 the eastern shores of our Continent, gives 

 rise to the prevalence of fog over the 

 Banks of Newfoundland. 



There is another atmospherical phenome> 

 non which, though it does not aff"ect the hy- 

 grometer, and is only indirectly connected 

 with moisture, is generally classed with fogs. 

 I alliide to what is called dry fog — a smoky 

 haziness of the atmosphere, which frequent- 

 ly extends over a large portion of the earth. 

 The nature of these fogs is bow pretty well 

 understood, and more refined observations, 

 particularly with the microscope, have ser- 

 ved to dissipate the mystery ia which they 

 wwe formerly enshrouded. When a portion 

 of the air in which the fog exists is filtered, 

 as it were, through water, and the substance 

 which is retained is examined with the mi- 

 croscope, it is founel to consist of minute frag- 

 ments, in some cases, of burnt plants, and ia 

 others of the ashes of volcanoes. It is sur- 

 prising to what a distance the pollen of 

 plants and minute fragments of charred 

 leaves may be carried. Sample-s of substan- 

 ces which have been collected from rain 

 water and examined microscopically by Pro- 

 fessor Schaefi"er, of Washington, at the re- 

 quest 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 subject. Dr. Small- 

 wood communicated a series of interesting 

 observations to the American Association at 

 their meeting in Albany, in 1855. Parti- 

 cles of matter of the kind we have descri- 

 bed are good absorbers and radiators of 

 heat, and hence in the daytime they must 

 become warmer than the surrounding atmos- 

 phere, and tend to be buoyed up by the ex- 



