674 
PROFESSOR JAMES THOMSON ON THE 
held at Dublin, in the following year, 1857 ; and a clear account of it is to be found in 
the Abstract of the paper published in the British Association volume for that year. 
The verbal explanations given in the reading of that paper before the meeting were 
illustrated by a drawing showing the scheme of circulation described in the paper. 
Fig. 4, here given, is an accurate copy of that drawing, differing from it only in some 
unimportant matters, such as in the number of arrows shown, and in its being drawn 
with abatement of some exaggerations which were made in the original in order to 
render small features more readily visible at a distance in a large room. The full 
significance of the original in all respects is retained unchanged in the copy here. 
In endeavouring to penetrate the mystery as to what the courses of circulation 
might be in the middle and higher latitudes, I was in preliminary ways fully satisfied 
that Hadley’s theory* in its main features—those, in fact, which in the present paper 
I have already described with commendation—must be substantially true, and must 
form the basis of any tenable theory that could be devised. 
Now, under Hadley’s theory, when we come to consider what may be the courses 
of circulation that we should attribute to the atmosphere in the latitudes outside ot 
the trade-wind zones, we should naturally be led to expect (as I have pointed out in 
some detail in an earlier part of the present paper in describing his theory) that the 
great sheet of air floating out from the Equator in the upper regions of the atmosphere 
towards either Pole, while having a motion towards the east also, would gradually cool 
in advancing to higher latitudes, and would therefore descend in middle and high 
latitudes to the Earth’s surface and would next, as a bottom current, flow back 
towards the Equator while also flowing eastward, and so would be a current towards 
the Equator, not towards the Pole. But, on the other hand, it had been brought out 
through accumulated observational results that the winds of middle latitudes while 
blowing towards the east, which so far is in agreement with Hadley’s theory, do, in 
opposition to what would be expected under that theory, blow more towards the Pole 
than from the Pole. Thus the facts and theory seemed to be at variance. It then 
occurred to me that facts and theory could be reconciled by supposing that the great 
circulation brought into probability under Hadley’s theory does actually occur, but 
occurs subject to this modification, that a thin stratum of air on the surface of the 
Earth in the latitudes higher than about 30°—a stratum in which the inhabitants of 
those latitudes have their existence, and of which the movements constitute the 
observed winds of those latitudes—being by friction and impulses on the surface of 
* Reference having been made in the text here to my paper read at the British Association Meeting 
for 1857, on the “ Grand Currents of Atmospheric Circulation,” and to the Abstract of it printed in the 
volume of the Association for that year, I have to mention as a correction that the theory here described 
and correctly designated as Hadley’s Theory, was in the printed Abstract erroneously named as 
Halley’s Theory. I was led into that mistake as to authorship of the commonly accepted explanation 
of the trade winds, through my finding it designated as “ Halley’s theory of the trade winds ” by Mauky', 
in his “ Physical Geography of the Sea,” to whose newly proposed views in that book my attention 
was at the time specially applied. 
