20 Mr. D. D. Heath on Mr. Fen-el's 
about the equator. Is this one of the results which please 
Dr. Haughton as conformable to facts ? 
Some other curious results might be deduced from his nu- 
merical formulas; but I will not dwell on them. Mr. Ferrel 
himself is conscious that all this is not very like nature, and 
so suggests "the resistance of the earth's surface" may con- 
siderably modify the state of things. And having once called 
in aid this agency, like the god on the Athenian stage, he is 
free from all restraint from formulas, and rearranges all his 
shapes and currents — for aught I know in fair conformity 
with facts, but with hardly any resemblance to his first picture. 
One would think that friction would rather hinder than help 
the air's spreading towards the poles. 
3. Another (a minor) point to observe is that, on commen- 
cing his investigations, he does not prove, but at once assumes 
that the upward and downward motions of the air, due to 
changes of temperature, which I suppose undoubtedly play a 
large part in these phenomena, are themselves everywhere 
insensible as wind. 
4. I should mention that in his Fourth Section, long after he 
has deduced these results from the supposition of absolute 
uniformity of temperature, he does make a supposition of its 
varying with latitude ; and from this, combined with deduc- 
tions from barometrical observations, he draws some inferences 
as to the strength of the average east or west components of 
the wind at different places and different heights. How near 
they may be to actual fact I do not know ; but I must warn 
my readers (not to go into more abstruse criticisms) that, no 
doubt from mere inadvertence, he works on the supposition 
that the hotter the air the smaller the pressure for a given 
density*, and, moreover, will not allow that the variation of 
temperature with the height of the strata can " produce any 
sensible effect" on the difference of velocity of the wind at 
different heights. 
I hope I have, so far, succeeded in showing to those I have 
in view that they had better trust to the old text-books than 
to Mr. Ferrel, as far as fundamental principles are concerned, 
though I dare say his store of facts may be greater. I will 
now say a few words about his use of his mathematical tools. 
Putting P for pressure, k for density, and H for the gravity 
potential, he begins, as Laplnce and Airy do, with the equa- 
tions in fixed rectangular coordinates -y-^ 4- 7— H — -»— =0, 
° at- dx k da 
* He puts k (density) = *P. and then supposes cc to vary directly as 
the "absolute temperature," usually written ( -\-t ) . 
