60 Mr Blackadder on the Formation of Dew. 
tion produced by the decreasing temperature of the lower air 
must produce a subsidence, especially in calm weather, of the su- 
perior body of air ; and if a cloud be suspended in the latter, 
the same appearance will be produced as if the cloud approach- 
ed nearer the earth by descending through the air. But as long 
as a cloud has the same temperature with its suspending me- 
dium, and is not operated upon by contending or modified cur- 
rents of air, it never approaches nearer to the earth, excepting 
in connection with the body of air in which it is suspended. 
It is a common opinion, that clouds at a distance from the 
earth are dissolved , sooner or later, when the body of air in which 
they are suspended is not saturated with moisture ; but it has 
never been explained how clouds are so commonly dissolved in 
the evening, when the air, by becoming colder and more dense, 
must be less disposed to receive an accession of moisture in the 
form of vapour. It is evident, indeed, that the mere dispersion 
of a cloud has been mistaken for its solution, or its union with 
air in the form of a pellucid vapour. The distinction, however, 
is both obvious and necessary. There is reason to believe that 
it is a rare occurrence for the air in the free atmosphere, to be 
in a state of perfect saturation ; and perhaps we have as yet no 
instrument that can exactly, and in all circumstances, indicate 
that state. It is pretty evident, however, that the particles of 
moisture congregated in the form of a cloud, very soon begin to 
separate, when the air in which they are suspended attains, and 
probably when it nearly approaches, to a state of saturation. It 
is possible, that, on such occasions, the cloud may acquire such 
a density, and the air may attain to a state of saturation with such 
a rapidity, that a deposition of rain may be produced. But it 
happens much more frequently, that the particles constituting 
the cloud are gradually dispersed through the air, in the form 
of a haze, and subside towards the surface of the earth ; and 
sometimes, especially towards morning, obscure the whole at- 
mosphere in the form termed a fog ; while part is deposited on 
the grass, and other bodies on the surface of the earth ; thus 
greatly increasing the secondary formation of dew, and damping 
all bodies indiscriminately that are freely exposed to the air. 
But, have we reason to believe that air, not saturated with 
