G60 
PROFESSOR JAMES THOMSON ON THE 
corresponding to the Sun in its apparent diurnal revolution, or in a world revolving 
on its axis as does the Earth, he shows that in the latter case—the case, namely, of 
the revolving Earth—in addition to such circulation as has just been described, east- 
to-west and west-to-east motions relative to the Earth’s surface would necessarily 
come into being for reasons which may be stated or suggested as follows :— 
If we consider the air in a nearly calm region at the outer limits of the trade-wind 
zone, and regard the air at that place as being at rest relatively to the Earth’s surface, 
and if we consider it to be drawn over the surface by indraught towards the Equator 
without application to it of any other force than that of the indraught, except what 
it may receive by friction from the surface of the Earth, be that land or ocean, this 
air in arriving at places always lower and lower in latitude (and consequently further 
and further out from the Earth’s axis) is coming to places in succession each moving 
eastward quicker than the previous one ; and thus the air is arriving successively at 
places each going quicker eastward than the air itself was going when at the previous 
place ; consequently the air in arriving at each new place must obviously have a slower 
motion eastward than the Earth’s surface at that place has. 
Thus throughout that course the Earth must be rushing forward under the air 
eastward quicker than the air goes, and that is the same as to say that the air 
must be blowing westward over the surface of the Earth. 
In connection with this part of his theory he brings into notice that, while the 
surface of the Earth at the outer edges of the Trade Winds has much less of absolute 
velocity eastward in diurnal revolution round the Earth’s axis than the surface at or 
near the Equator has, yet the trade-wind air, on arriving at the foot of the equatorial 
belt of rising air after its course from those outer parts in higher latitudes, has become 
imbued with eastward velocity little less than that of the equatorial surface of the 
Earth, the only deficiency in this eastward velocity from that of the equatorial 
surface being what is manifested as wind blowing westward over the Earth’s surface, 
or having in relation to that surface a moderate westward velocity. He shows, for an 
example, that the eastward velocity of the Earth at either of the tropic circles is less 
than that at the Equator by about 87 miles per hour, but yet that the air which comes 
from calm regions near the tropic circles to the equatorial belt has, on its arrival at 
that belt, an eastward absolute velocity which is only a few miles per hour in defect 
of the velocity of the Earth there, the actual defect being manifested in the relative 
velocity with which the wind at the equatorial parts blows westward over the surface 
of the land or sea. He explains that this result is brought about by reason that the air, 
during its course from the outer edge of the trade-wind zone to the foot of the equa¬ 
torial rising belt, is perpetually being dragged forward eastward by the quicker-moving 
land or sea below it, and so its velocity is kept nearly assimilated to that of the part 
of the Earth over which, for the time being, it exists, and is allowed only to be a 
little less than that velocity. 
Such, then, is Hadley’s theory, in so far as it relates to the origin of the Trade 
