390 
VOYAGE OF THE POTOMAC. 
[May, 
The cause of the tradewind, so remarkable on account of its 
general uniformity everywhere between the tropics, is principally 
owing to the joint influence of the superior temperature of the 
torrid zone, and the rotation of the earth on its own axis. The 
air at the surface of the earth being highly rarefied between the 
tropics, naturally ascends by its acquired levity into the higher 
regions of the atmosphere ; while its place is supplied by the 
colder air rushing from the poles towards the equator. This new- 
comer soon becomes rarefied like its predecessor, and ascends in 
its turn, and is ultimately carried towards the two poles, to 
supply the deficiency caused by the continual stream from these 
points. 
An aerial current is thus established, constantly proceeding 
from the poles towards the equator, in the lower regions. Each 
of these polar currents, moving in its progress towards the equator, 
from zones where the earth's motion on its axis is slower, to others 
where it is more rapid, cannot have the same velocity eastward 
as the solid parts of the globe ; and, therefore, the aerial parti- 
cles of which it is composed, gradually acquire a relative motion 
in an opposite direction. The currents from the two hemispheres 
meeting near the equator, and verging, each of them, towards the 
west, the meridional motions are destroyed by being in opposite 
directions ; and they therefore advance together, with the remain- 
ing motion from the eastward, all round the globe. ^ 
But there are certain situations, where the tradewind is not 
only interrupted in its general course towards the west, but is ac- 
tually suspended, and succeeded by a wind blowing in an opposite 
direction, which continues with great regularity for several months. 
This is classed under the general term of periodical winds ; and 
is called a monsoon, from the Persian word monsum, which sig- 
nifies season. In the East Indies, there are two of these peri- 
odical winds, distinguished by the names of the northeast and the 
southwest monsoons; which may be said to change every six 
months, according to the sun's place in the ecliptic. In the same 
class are also included the sirocco, the simoon, land and sea- 
breezes, and long-shore winds. The sirocco is a periodical wind, 
which prevails in Italy about Easter; and the simoon is a hot, 
suffocating wind, which prevails at certain seasons on the deserts 
of Arabia and Africa. Both of these winds are supposed to be 
