WINDS. 
391 
1832.] 
highly charged with carbonic acid gas, mixed with some other 
noxious exhalations, unsuitable for respiration. 
Perennial winds are those which blow the whole year in one 
direction ; while those which are called variable, are those which 
prevail in all parts of the world beyond the tropics. In the middle 
of the Atlantic Ocean, between the two great continents, Africa 
and America (and a very few degrees east and west of that 
central meridian), the regular perennial winds constantly prevail, 
subject only to shght variations from the position of the sun. 
While the sun is passing over the equator, it is often difficult 
for vessels to cross the line ; and such is the influence of that 
luminary, that they are sometimes becalmed until his decHnation 
increases to seven or eight degrees. But when at fifteen degrees, 
particularly if near either of the tropics, they generally pass the 
line with a fresh breeze. At these times, the tradewind also 
varies. When the sun is in Cancer, the southeast perennial 
extends from four to six degrees across the line to the northward, 
and inclining more to the south than to the east ; and the contrary 
takes place when the sun is in Capricorn. This is a natural con- 
sequence. The lower current of air being rarefied by the reflected 
heat of the sun, ascends, and the equilibrium is restored by a 
larger body of dense air, which rushes forward in a right hne, and 
with a strong current, to fill up the vacuum. 
The ancient Portuguese, not being acquainted with these cir- 
cumstances, thought to shorten the route of their East India 
voyages by hugging the continent of Africa ; an error which 
inevitably exposed them to calms and storms, and sometimes 
prolonged their voyages to an extent of two and three years. 
Long and dear-bought experience, however, has since taught 
navigators to hold nearly a middle course between the two conti- 
nents, in which they are favoured by constant tradewinds, and 
where they may allow some leeway in the southern tropics, a 
precaution necessary when the sun is in or near the tropic of 
Cancer ; for at that time the southeast wind inchnes very much to 
the south. Both outward and homeward bound ships, engaged 
in the India trade, should therefore cross the equator in about 
eighteen or twenty degrees of west longitude ; by which means 
they will not fall in with the American continent, and, at the same 
time, will avoid the calms on the coast of Africa. 
