GRAND CURRENTS OF ATMOSPHERIC CIRCULATION. 
G77 
sitions an idea which, taking it in a wider scope than that in which he applied it, and 
with congruity in application not pertaining to the case for which he adduced it, I 
may describe as implying considerations to the effect that in an atmosphere covering 
a zonal region such as that of the middle latitudes, and having eastward motion 
relative to the Earth’s surface or, what is the same, having a speed of eastward 
revolution quicker than that of the Earth below it, a layer at bottom retarded 
by friction on the Earth’s surface, and so having less centrifugal tendency than has 
the quicker eastward-going air above will be caused to take, along with its eastward 
motion, a motion also towards the Pole. 
The principle is an important one in its applicability to atmospheric circulation ; 
but Mr. Ferrel did not apply it to good account. He applied it only in reference 
to a system of motions already assumed by him, but which in the actual atmosphere 
are impossible as to causes for their origin and maintenance, and are incongruous in 
their mutual relations. His purpose in this matter was to show reason for the 
bottom current flowing towards the Pole while he had the upper current assumed 
as flowing towards the Equator. He assumed throughout the whole depth from 
bottom to top in his zonal ring of the atmosphere a motion eastward relative to the 
Earth, and thereby explained that the frictionally resisted bottom part should flow 
towards the Pole. But now we have to observe that the only reason why under his 
theory he can be entitled to assume eastward motion in the lower portion is because 
of that portion having been previously assumed to flow towards the Pole ; and as to 
the upper portion which he assumes to flow from the Pole, that reason does not hold 
at all, and the upper portion should rather be supposed, under his theory, to flow 
westward than eastward. Thus it comes out that he explained the motion towards 
the Pole in the lower part of the atmosphere by first assuming, for no valid reason, a 
motion towards the Pole of that lower part. But now, for the primary assumption of 
that motion towards the Pole in the lower portion of the atmosphere, the reason 
which he assigned, and which I have just now treated as being not valid, was his 
supposed heaping up of the atmosphere at top, and consequent increased pressure at 
bottom at about the parallel of 28 °; but, for the heaping up of the atmosphere there 
he needs in the upper region of the atmosphere over the middle latitudes a speed of 
revolutional motion greater than that of the Earth’s surface immediately below, briefly 
a relative eastward motion, so that there may be the necessary centrifugal tendency 
for producing the heaping up, and that is incongruous with the flow in those upper 
regions taking place, as under his theory he made it do from higher to lower lati¬ 
tudes—from the Arctic Circle to about the parallel of 28 °. 
He has not thereby anticipated the new and, I think I may say, the true theory 
offered by me, in which the great body of the lower half of the atmosphere is already 
shown for good reason to have motion towards the Equator along with motion from 
west to east, but that a comparatively thin lamina at bottom of it, in virtue of fric¬ 
tional retardation of its eastward motion and consequent abatement of centrifugal 
