We now come to the causes which are auxiliary and interfering. It is
natural that we should regard the sun as the first and most influential
of these causes, as being the source of that variation in the
temperature of the globe, which alternately clothes the colder regions
in snow and verdure. The heat of the sun undoubtedly causes the ether of
the lower atmosphere to ascend, not by diminution of its specific
gravity; for it has no ponderosity; but precisely by increase of
tension, due to increase of motion. This aids the ascensional movement
of the air, and therefore, when a vortex is in conjunction with the sun,
its action is increased--the greatest effect being produced when the
vortex comes to the meridian a little before the sun. This has a
tendency to make the period of action to appear dependent on the phases
of the moon, which being the most palpable of all the moon's variations,
has been naturally regarded by mankind as the true _cause_ of the
changes of the weather. Thus Virgil in his Georgics, speaking of the
moon's influence and its signs:
"Sin ortu in quarto (Namque is certissimus auctor)
Pura, nec obtusis per coelum cornibus ibit;
Totus et ille dies, et qui nascentur ab illo,
Exactum ad mensem, pluvia ventisque carebunt."
Hence, also, in the present day we hear sailors speak of the full and
change, or the quartering of the moon, in connection with a gale at sea;
thus showing, at least, their faith in the influence of the phenomenon.
Yet it is actually the case, at certain times, that in about latitude
40d and 41d, the storms appear about a week apart.
There is some reason, also, to suspect, that there is a difference of
temperature on opposite sides of the sun. As the synodical rotation is
nearly identical with the siderent period of the moon, this would
require about forty-four years to run its course, so as to bring the
phenomena to exact coincidence again. Since these observations were
made, it is understood that Sig. Secchi has determined that the
equatorial regions of the sun are hotter than his polar regions. It may
be owing to this fact, that we have inferred a necessity for a change,
whose period is a multiple of the sun's synodical rotation, but it is
worthy of examination by those who possess the necessary conveniences.
Another period which must influence the character of different years,
depends on the conjunction of the perigee of the lunar orbit with the
node. Taking the mean direct motion of the moon's perigee, and the mean
retrograde motion of the node, we find that it takes six years and one
day nearly from conjunction to conjunction. Now, from the principles
laid down, it follows, that when the perigee of the orbit is due north,
and the ascending node in Aries, that the vortices of the earth will
attain their greatest north latitude; and when these conditions are
reversed, the vortices will reach their highest limit in the lowest
latitude. This will materially affect the temperature of the polar
regions. In the following table, we have calculated the times of the
conjunctions of the apogee and pole of the orbit, taking the mean
motions. It may be convenient to refer to by-and-bye, remembering that
when the conjunction takes place due south, the vortices reach the
highest, but when due north, the vortices in the northern hemisphere
have their lowest upper limit: