Theory of Atmospheric Currents. 15 
space, in a straight line, in a direction and with a velocity 
compounded of the velocity it had in common with the sur- 
face at that place and that of the impulse given to it, in ac- 
cordance with the principle of the Parallelogram of Veloci- 
ties, and will be drawn out of that line 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 comprises the original direction and this centre. From 
this it will be seen that, if started due north from the equator 
with a velocity of even 1000 miles an horn- — the same as the 
eastward velocity of the surface there — it will only reach the 
latitude of 45°. Up to that culminating point it will gain 
apparent or relative velocity, compared with the surface 
it is passing over, as explained in Mr. Ferrel's " text- 
book," and then will begin its descent, losing relative velo- 
city, cross the equator at the point opposite to the one it 
started from, and complete its circuit in the other hemisphere, 
just as the moon does at a greater distance — that is, supposing 
gravity does not, long before then, pull it down to the earth's 
surface. This effect of equable motion combined with passage 
over different latitudes is precisely one part of the explanation 
of that variability of the sun's apparent motion which gives 
rise to the " Equation of Time/' or difference between solar and 
clock time. 
Sir J. Herschell, I may remark, in his ' Meteorology,' does 
not thus mislead his readers. He takes the lower current of 
cold air from the north, and traces it downwards towards the 
equator; and this, if unimpeded, it will reach with ever so 
little original impulse southward; and even with the actual 
resistance the impelling force arising from the difference of 
temperature will always be helping, and may prevail. When 
he comes to consider the upper northward current, he only 
conducts it " beyond the tropics," and there brings it down 
to earth. 
I have gone at length into this very elementary exposi- 
tion, because a clear conception of the facts will prevent a 
great deal of confusion from Mr. Ferrel's papers. But this is 
by no means the objection which he in this place goes on to 
bring against the " mischievous " teaching of the text-books. 
TThat he says, after the passage I have quoted, is:— "•But 
this is at variance with a fundamental and well-established 
principle in mechanics. The force in this case is a central 
force j or at least the compound perpendicular to the earth's 
axis [sic, I do not understand what resolution of the force he 
has in his mind] can be neglected, since it can have nothing to 
do with any east or west motion. This being the case, the 
