Theory of Atmospheric Currents. 19 
with no surface friction, any motion once started might indeed 
alter its character, but would never cease, and would blend 
with and affect the permanent phenomena. But with fluids 
and surfaces as they are, each such accidental disturbance will 
sooner or later be exhausted; and we may reasonably endea- 
vour to find what minimum of motion is consistent with the 
existence of the permanent forces, and assume that this will 
be the average state. This Mr. Ferrel himself states. Now, 
as he takes no notice of tides, these permanent causes of 
motion are gravity and the unequal action of the sun's heat, 
according to season, latitude, and longitude (as fixing the time 
of day and night at each place), and height above the surface. 
And gravity of itself requires no permanent relative motion; 
for the ' Theory of the Figure of the Earth ' shows that, where 
there is no inequality of temperature, the fluids can arrange 
themselves in a permanent shape adapted to a uniform rota- 
tion round the earth's axis. There will be relative motion of 
the parts until this shape be reached; afterwards, relative rest. 
Mr. Ferrel accordingly begins with supposing the heat does 
vary according to latitude, longitude, and height, though he 
takes no notice of seasons. But he is quite entitled to say the 
complete problem is too difficult, and to simplify it down as 
far as he thinks it necessary. If he could give us a satisfac- 
tory solution on the supposition that the localized and moving 
sun was replaced by a uniform equatorial ring of heat, I 
suppose he would have performed a great feat. But he does 
not start in any such way. He at first supposes the tempe- 
rature to vary somehow: for aught that appears in his formula, 
the torrid zone might be the coolest, and the polar regions the 
warmest portions of the earth, and the air hotter 7 miles high 
than at the surface. But when he begins to get results 
(Sect. II.), he first supposes temperature to vary only with 
latitude, allowing no difference for height, and finally " sup- 
poses the density," and of course the temperature with it, 
" to be independent of the latitude." But this supposition 
amounts, as I have already pointed out, to supposing that 
there is no force at all to sustain any motion, and that conse- 
quently there are no permanent tendencies in the winds — that 
the average state is one of relative rest, the air arranging 
itself in conformity to the Theory of the Figure of the Earth. 
Thus all his work should be all to no purpose ! 
2. But he fancies he does get results. And this is the 
principal one: — "The fluid, however deep it may be atthe Equator, 
cannot exist at the Poles.'''' And he draws a picture of the atmo- 
spnere as it should be, banked up from somewhere near the 
arctic circle towards the middle latitudes, but with a depression 
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