-1859] WRITINGS OP JOSEPH HENRY; 255 



ing filled with a mass of fog, exhibiting a definite and level 

 surface presenting the appearance of a lake the shores of 

 which are the surrounding eminences; and if a depression 

 of sufficient depth occurs 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 applicable to the phenomenon fre- 

 quently observed of early frost in the same localities. As 

 rapidly as the air is cooled on the sides of sloping ground it 

 sinks into the valley below and its place is supplied by 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 oolder than the more ele- 

 vated parts of the surrounding surface. Fogs are produced 

 on the ocean when a gentle wind charged with moisture 

 mingles with another of a lower temperature. The wind 

 from the Gulf Stream mixing with the cold air which rests 

 upon the water from the arctic regions, (which as before 

 stated flows along close to the eastern shores of our con- 

 tinent,) gives rise to the prevalence of fog over the Banks of 

 Newfoundland. 



There is another atmospherical phenomenon which though 

 it does not affect the hygrometer and is only indirectly con- 

 nected with moisture, is generally classed with fogs. I allude 

 to what is called dry fog — a smoky haziness of the atmos- 

 phere, which frequently extends over a large portion of the 

 earth. The nature of these fogs is now pretty well under- 

 stood, and more refined observations, particularly with the 

 microscope, have served to dissipate the mystery in which 

 they were formerly enshrouded. When a portion of the air 

 in which the fog exists is filtered through water and the sub- 

 stance which is retained is examined by the microscope it 

 is found to consist of minute fragments, in some cases of 

 burnt plants, and in others of the ashes of volcanoes. It is 

 surprising to what a distance the pollen of plants and minute 

 fragments of charred leaves may be carried. Samples of sub- 

 stances which have been collected from rain water and ex- 



