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towards the east. In like manner western winds blow on 
the eastern side of the Pacific ocean within both the northern 
and southern tropics, and must therefore rotate eastward 
more swiftly than the surface on which the air presses. It 
was contended that the cause which determined the air con- 
stituting these winds to rotate faster than the surface of the 
globe that was beneath them, ought to be considered that 
which makes other winds pass over the surface when they do 
not move with equal rotatory velocity, and are therefore said 
to be left behind. We cannot presume that there are two sepa- 
rate causes in operation to create these winds — one making 
them move faster than the surface which supports them, and 
the other slower. In the Indian ocean also, as well as 
among the islands of the great East Indian Archipelago, 
strong evidence on the subject is to be found. Thus the 
north-western monsoon, and other similar winds, are not left 
behind by the more rapidly rotating equatorial surface ; and 
there does not appear to be any reason to doubt that they are 
made to rotate faster than the surface over which they are 
passing by the same cause which produces all other winds, 
including those -which are said to be produced by being left 
behind. It was also pointed out that in both the Pacific 
and Indian oceans, regular season winds blow ; in one season 
eastward, making the air pass in that direction more rapidly 
than the surface of the globe, and at another season westward, 
when the air must move less rapidly than the surface revolves, 
and is therefore left behind ; but the rotation of the surface 
can no more be supposed to retard one of these winds than 
that it can accelerate the speed of the other. It was contended 
that the cause of all such winds is to be found in atmospheric 
vacua, created by condensation of aqueous vapours and 
consequent heating and expansion of the gases where the 
heating takes place. This expansion creates a partial vacuum 
in the atmosphere, into which neighbouring air rushes to 
restore the disturbed equilibrium of pressure ; and it is this 
rush of air that constitutes all winds, whether they move in a 
