Theory of Atmospheric Currents. 2G5 
space, in a straight line, in a direction and with a velocity 
compounded of the velocity it had in common with the surface 
at that place and that of the impulse given to it, in accordance 
with the principle of the Parallelogram of Velocities, and will 
be drawn out of that hue only downwards, by the force of 
gravity at the centre of the earth : and therefore it will never 
leave the plane of the great circle, fixed in space, which com- 
prises the original direction and this centre." This refers to 
absolute motion, while the question is one of relative ; it will 
be spoken of further on. However, that this absolute motion 
is not in the least contradictory to the application of the prin- 
ciple of the preservation of areas as carried out by Ferrel and 
others and as questioned by Mr. Heath, is proven by Dr. A. 
Sprung in an article, " Zur Anwendung des Princips der 
Flachen in der Meteorologie," Oesterr. Zeitschrift fur Meteo- 
rologie, Band xvi. page 62. 
On page 15 he remarks that Sir John Herschel, in his 
1 Meteorology,' does not mislead his readers as Ferrel does. 
It may be here remarked that, in the book referred to, Her- 
schel did not give the true theory of the winds as at present 
accepted. Ferrel was right in speaking of the wrong teach- 
ing of the text-books, because it is only a few years since the 
true explanation was first given in any of them, and even 
now those in which it is found can be counted on the fingers 
of one hand. 
The " force"'' which is described on page 15 as incomprehen- 
sible is explained in Poisson's paper od projectiles, Cl Memoire 
sur le mouvement des projectiles dans 1' air, en ayant egard 
a la rotation de la terre," lu a l'Acadeinie des Sciences, le 14 
novembre 1837, Journal de V Ecole Poly technique, cahier xxvi. 
There is also a discussion on the subject in the Cornptes JRen- 
dus, tome xlix. &c, etc. ; and Poisson's paper is mentioned in 
Routlx's ' Rigid Dynamics ' (3rd edition), 1877, p. 218. 
In regard to Ferrel's criticisms of Colding, he was right 
in saying that Colding did not take into account one compo- 
nent of the effect of the earth's rotation. 
As to Ferrel and Everett having jointly discovered this 
component, there has never, to my knowledge, been such a 
claim made. It was discovered many years previously, but was 
rediscovered by Ferrel independently. 
In regard to the two points brought out on page 18 — first, 
the application of the principle of the conservation of areas, 
and, secondly, the path of a body on the earth's surface — Mr. 
Heath is in error in his criticism. As to the first point, this is 
not the first time that the application of this theorem has been 
questioned. Dr. Thiesen employs it in an article " Ueber 
