Theory of Atmospheric Currents. 21 
and so on, making the true remark that, "putting P = 0, 
they are the equations to a projectile." 
And he might hare observed that (assuming symmetry 
round the axis of rotation) these equations are the same 
whether the earth is revolving or not, and that, when we 
put P = 0, we get the path of the projectile in fixed space the 
same for a stone or for the moon. 
He then, as Laplace and Airy do, transforms the equations 
for polar coordinates. And if he and they retained, for a 
while, id for the fixed celestial longitude, we should have 
equations equallv independent of rotation. Writing —g for 
-T-, they would be 
ldP dV 
k dr ~ dt 2 
lfZP 9 <P0 _ drdd . . . a (dco\ 2 
«dd=- r 'd¥ - 2r dldF +r ~ smecose {w) > 
1 <*? = _ r2 (sin 6f^-2r (sin 0)-% £ 
Kdco K } dt* v } dt dt 
9 . a Q dd dco 
— Ir sin a cos o^ — =-• 
dt dt 
But when we put o) = nt + (f), we get equations adapted to a 
terrestrial, revolving meridian. And when we make n = the 
actual angular rotation, and suppose the figure of the earth, 
with the undisturbed sea and atmosphere, such as are adapted 
for keeping the whole system in relative rest (see Airv, para- 
graph 79), then we can separate what belongs to the undis- 
turbed system and what to the motions, shapes, and pressures 
arising from the extraneous forces (here those due to change 
of temperature). And then the terms in question make their 
appearance. Mr. Ferrel makes, substantially, the same sim- 
plifications which Laplace and Airy make in the Tidal theory. 
They do not seem quite so legitimate here*, where there is no 
a priori ground for supposing the motions very small ; but I 
am not quarrelling with this. His next, his first original 
step is to lay it down that " For a stratum of equal pressure 
P is constant, and hence ~jn =0 and — =0;" and of course, 
f?P 
bv parity of reasoning. -— =0. And all his formulas, I be- 
• ° 7 dr ' 
lieve without exception, till he comes to those in the Fourth 
* In my paper I ventured to suggest some doubts on the subject even 
as regards tidal motion near the poles. 
