in a Voyage across the Atlantic. 127 
warmer, or it may be colder and damper; provided the excess of 
dryness in one case, and that of coldness in the second, surpass 
the superabundance of warmth and of moisture respectively. 
Wind, passing from a frozen region to a more temperate one, 
is an instance of a current of cold and dry air. 
A parched hot wind, such as blows off a heated desert, is an 
example of an atmospheric current, wherein excess of dryness 
overpowers deficiency of cold. 
Cold damp winds, such as blow from the sea upon the shore, 
exemplify currents of air, in which excess of cold outdoes defi- 
ciency of dryness. 
Land and sea breezes, alternating upon coasts in warm cli- 
mates, are likewise instances of dry but hot wind, and cool but 
damp. When excess of dryness above warmth in the atmo- 
sphere over the land, surpasses that of cold above moisture in 
the air over the sea, a land-breeze blows: when excess of cald 
above humidity upon the sea outgoes that of dryness above heat 
on shore, the sea breeze sets in. 
Warm and damp air, being lighter than that which is either 
cool or dry, can be but an ascending current, and may be felt 
as wind upon an acclivity or elevation. A descending current 
of air, as well as an ascending one, is ordinarily referred by the 
feelings to a horizontal direction. The impulse of the wind is, 
in either case, oblique: but, in the sensation of it, a moderate 
degree of slope is not distinctly perceptible. An ascending 
current then may be wind, in like manner as a descending one 
certainly often is so. A puff of air from a passing cloud, and a 
gust from a neighbouring mountain, are, no doubt, descending 
currents, though their slanting direction may be unobserved. A 
puff of warm air is not unfrequently felt on the side of a hill, 
unquestionably coming from the valley below ; and a fog is very 
commonly seen rising from the sea towards a high coast. 
A trade-wind, passing from a cool and dry, towards a warm 
and damp, region, may comprise. descending currents: of air, 
which feed its continued progress, At its outset, it seems to be 
indubitably such. Thus, in the northern Atlantic, from the 
