[Illustration: FIG. 27.--The Changes of the Seasons.]
In the adjoining figure, I show a little sketch (Fig. 27), by which I
try to explain the changes of the seasons. It exhibits four positions
of the earth, one on each side of the sun. The left. A, represents the
earth when summer gladdens the northern hemisphere; while the right,
C, shows winter in the same region. You will see the two central lines
which represent the axis about which the earth rotates. Of course,
the earth has no visible axis. The line which runs through the globe
from the North to the South Pole is imaginary. It remains fixed in the
earth, for we can prove in our observatories that the Pole does not
shift its position to any considerable extent in the earth itself.
In fact, if we could reach the North Pole and drive a peg into the
ground year after year to mark the exact spot, we should find that
the position of the Pole was sensibly the same. Does it not seem
strange that we should be able to know so much about the Pole, though
we have never been able to get there; have never, in fact, been able
to get within less than 400 miles of it? I think you will be able to
understand the point quite easily. The latitude of a place, as you know
from your geography, is the number of degrees, and parts of a degree,
between that place and the equator. In our observatories, we can
determine this so accurately that the difference between the latitude
of one side of a room and of the other side of the same room is quite
perceptible. As we find that the latitudes of our observatories remain
sensibly unchanged from year to year, we are certain that the Pole
must remain in the same place. Indeed, if the Pole were to alter its
position by the distance of a stone’s throw, the careful watchers in
many observatories would speedily detect the occurrence.
And now I must direct your attention to something apparently quite
different. When the battle of Waterloo was fought, the great victory
was won with the aid of the old-fashioned musket, a smooth-bore gun
which was loaded at the muzzle with a good charge of powder, and then a
round bullet was rammed down. “Brown Bess,” as the musket was called,
was a most efficient weapon at close quarters, and indeed at any
distance _when the bullet hit_; but there was the difficulty. The round
bullets, rushing up the tube and out into the air in a somewhat vague
manner, had a habit of roaming about, which was quite incompatible with
the accurate shooting of our modern rifles.
One great improvement in small arms consisted in giving to the bullet a
rapid rotation about an axis which is in the line of fire. This is what
the _rifle_ accomplishes. The grooves in the barrel of the rifle twist
round, and though they only give half a complete turn in the length of
the barrel, yet the speed of the bullet is so great that when it flies
off it is actually spinning with the tremendous velocity of about one
hundred and fifty revolutions a second. Even with the old-fashioned
round bullet, the rifling of the barrel effected great improvement in
the accuracy of the shooting. The introduction of the elongated bullets
was another great improvement, while the adaptation of breech-loading
enabled a bullet to be used rather larger than that which could have
been forced down the barrel, and thus it was insured that the grooves
should bite into the bullet as it hurries past and impart the necessary
spin.
A body rapidly rotating about an axis has a tendency to preserve the
direction of that axis, and powerfully resists any attempt to change
it. Our earth is spinning in this fashion. It is true that the rotation
is, in one sense, a slow one, for it requires almost an entire day for
each rotation. But when we remember the dimensions of our earth, we
shall modify this notion. We have already stated that any place on the
equator has to travel more than one thousand miles each hour in order
to accomplish the journey within the required time. So far, therefore,
the earth moves like a rifle-bullet, and the direction of its axis
remains constant.
In the course of the great voyage between summer and winter, the earth
travels from one side of the sun to the opposite side, and in doing
so it still continues to spin about an axis parallel to the original
direction. See the consequences which follow. The sun illuminates half
the earth, and in the left position in Fig. 27, representing summer,
the North Pole is turned over towards the sun, and lies in the bright
half of the earth. There is continual day at the North Pole, and night
is unknown there at this time of year, because the turning of the earth
about its axis will not bring the Pole nor the regions near the Pole
into the dark hemisphere. Thus it is that the Arctic regions enjoy
perpetual day at this season. Look now at the position of England when
the northern hemisphere is tilted towards the sun, and is consequently
enjoying the full splendor of midsummer. As the earth turns round,
England will gradually cross the boundary between light and shade, and
will enter upon the darkened hemisphere. Then there will be night in
England, but you will see from the figure that the day is much longer
than the night, and hence it is that we enjoy the fine long days in
summer.
We next look at a different scene six months later. The earth has
reached the other side of the sun, but the axis has remained parallel
to itself, consequently the North Pole is now inclined entirely away
from the sun. The earth continues to turn round as before, but its
movements do not bring the North Pole or the surrounding Arctic regions
out of the dark hemisphere, and consequently the night must be unbroken
in these dismal circumstances. The long continuous day which forms the
Polar midsummer is dearly purchased by the gloom and cold of a winter
in which there is no sun for many weeks in succession. Observe also the
changed circumstances of England. In the course of each twenty-four
hours it lies much longer in the dark half of the earth than in the
bright, and consequently there is only a short day succeeded by a long
night.