238 Mr Joseph J. Murphy on Revolving Storms. 
monsoons. ‘They are certainly not formed in the South 
Atlantic, and we have no evidence of their existence in the 
Pacific, except perhaps off the coast of California. 
They are formed only over tropical seas, because there 
only the air contains watery vapour enough to furnish their 
motive power by its condensation. But they originate in 
eddies formed in the general circulation of the atmosphere. 
The heated air that rises up during summer over the 
warm regions of Asia and Africa flows out over the Atlantic 
as an upper current, where it meets the upper return trade 
wind. The direction of the former is nearly towards W., that 
of the latter nearly towards N.H., so that the resulting current 
will be N.N.W.; and on its right-hand side, towards Europe, 
eddies will be formed rotating with the hand of a watch, and 
on its left hand, or West Indian side, will be eddies rotating 
in the opposite direction. Any one may make such a pair of 
eddies by moving a teaspoon forward through a cup of liquid. 
At the centre of every eddy is a depression, produced by cen- 
trifugal force, and in an aerial eddy this will be marked by a 
diminution of barometric pressure. When the eddy rotates 
in the opposite direction to that of the cyclones of the 
hemisphere in which it occurs, no cyclone can be produced ; 
but when it rotates in their direction, and other circum- 
stances are favourable, the barometric depression at the 
centre produces cold—the cold of diminished pressure: this 
condenses part of the vapour in the air: heat is liberated, 
and an upward current produced, as explained at the begin- 
ning of this notice: an outflow of air takes place above 
and an indraft below: these radial motions combine with 
the rotatory motion of the eddy, and thus the eddy is con- 
verted into a cyclone. 
The cyclones of the Indian Ocean west of India are 
produced in exactly the same way, by the meeting of the 
upper currents from the heated regions of Asia and Africa: 
and those of the South Indian Ocean, by the upper current 
from the heated regions of Australia meeting the upper 
return trade wind. 
In these cases the eddy is formed originally in the upper 
strata of the air. In the Chinese Sea and the Bay of Ben- ~ 
gal, on the contrary, it is formed on the surface of the sea, 
eae 
