Mr Blackadder on the Formation of Dew. 57 
evaporation, the lower air is never free of aqueous particles, such 
as constitute a mist or fog. In all countries, the surface of the 
earth, when viewed at a distance, appears enveloped in a haze, 
of greater or less density ; and though, when sailing, for ex- 
ample, on the Pacific Ocean, the atmosphere appears very clear, 
and the sky brilliant, it is found, on being viewed from a high 
mountain, that the former is constantly obscured by a haze of 
considerable density, and which extends many feet above the 
surface. During the formation. of dew, when the general at- 
mosphere appears clear, and objects at a moderate distance seem 
as if surrounded by a perfectly transparent air, aqueous particles, 
such as constitute a haze, are never altogether absent; the 
quantity of these particles, and the greater or less rapidity of 
their formation depending on the particular state of the air in 
regard to moisture, as influenced by temperature. When they 
are in such quantity as to produce an indefinite obscuration of 
the air near the surface, they constitute a haze. When the ob- 
scuration is more complete, with a defined outline, from the par- 
ticles congregating in the form of a cloud, and either resting on 
the surface, or elevated a few feet above it, it is termed a mist. 
And when the whole atmosphere appears equally obscured, it is 
called a fog. This last is sometimes seen shortly after sunset, 
but most commonly makes its appearance towards morning; and 
has on some occasions been ascertained to extend to the height of 
from 50 to 100 feet above the ground ; its upper surface being 
horizontal, well defined, and having the appearance of a dense 
white cloud. This fog, however, must be distinguished from 
that obscuration of the air which frequently, and at different 
seasons, accompanies easterly and westerly winds, and which 
would require a separate investigation. 
It has already been observed, that both before and after dew 
has been deposited on a grassy surface, evaporation goes forward 
,not only at the surface of the blades of grass, but also from that 
of the subjacent soil; and it was also observed, that though the 
temperature of the latter gradually diminishes after sunset, it 
always remains higher than that of the air at some distance above 
the ground, and consequently must be considerably above that 
of the grass, and of the air in contact with it. As heat promotes, 
it follows that a loss of heat will retard evaporation ; more espe- 
