Chapter XXII.
"Beside the Moldau's rushing stream,
With the wan moon overhead,
There stood, as in an awful dream,
The army of the dead."Longfellow.
Most of our readers will understand what was meant by Mary Pratt's "inclination of the earth's axis to the plane of its orbit;" but as there may be a few who do not, and as the consequences of this great physical fact are materially connected with the succeeding events of the narrative, we propose to give such a homely explanation of the phenomenon as we humbly trust will render it clear to the most clouded mind. The orbit of the earth is the path which it follows in space in its annual revolution around the sun. To a planet there is no up or down, except as ascent and descent are estimated from and towards itself. In all other respects it floats in vacuum, or what is so nearly so as to be thus termed. Now, let the uninstructed reader imagine a large circular table, with a light on its surface, and near to its centre. The light shall represent the sun, the outer edge of the circle of the table the earth's orbit, and its surface the plane of that orbit. In nature there is no such thing as a plane at all, the space within the orbit being vacant; but the surface of the table gives a distinct notion of the general position of the earth as it travels round the sun. It is scarcely necessary to say that the axis of the earth is an imaginary line drawn through the planet, from one pole to the other; the name being derived from the supposition that our daily revolution is made on this axis.
Now, the first thing that the student is to fix in his mind, in order to comprehend the phenomenon of the seasons, is the leading fact that the earth does not change its attitude in space, if we may so express it, when it changes its position. If the axis were perpendicular to the plane of the orbit, this circumstance would not affect the temperature, as the simplest experiment will show. Putting the equator of a globe on the outer edge of the table, and holding it perfectly upright, causing it to turn on its axis as it passes round the circle, it would be found that the light from the centre of the table would illumine just one half of the globe, at all times and in all positions, cutting the two poles. Did this movement correspond with that of nature, the days and nights would be always of the same length, and there would be no changes of the seasons, the warmest weather being nearest to the equator, and the cold increasing as the poles were approached. No where, however, would the cold be so intense as it now is, nor would the heat be as great as at present, except at or quite near to the equator. The first fact would be owing to the regular return of the sun, once in twenty-four hours; the last to the oblique manner in which its rays struck this orb, in all places but near its centre.
But the globe ought not to be made to move around the table with its axis perpendicular to its surface, or to the "plane of the earth's orbit." In point of fact, the earth is inclined to this plane, and the globe should be placed at a corresponding inclination. Let the globe be brought to the edge of the table, at its south side, and with its upper or north pole inclining to the sun, and then commence the circuit, taking care always to keep this north pole of the globe pointing in the same direction, or to keep the globe itself in what we have termed a fixed attitude. As one half of the globe must always be in light, and the other half in darkness, this inclination from the perpendicular will bring the circle of light some distance beyond the north pole, when the globe is due-south from the light, and will leave an equal space around the opposite pole without any light at all, or any light directly received. Now it is that what we have termed the fixed attitude of the globe begins to tell. If the north pole inclined towards the orbit facing the rim of the table, the light would still cut the poles, the days and nights would still be equal, and there would be no changes in the seasons, though there would be a rival revolution of the globe, by causing it to turn once a year, shifting the poles end for end. The inclination being to the surface of the table, or to the plane of the orbit, the phenomena that are known to exist are a consequence. Thus it is, that the change in the seasons is as much owing to the fixed attitude of the earth in space, as we have chosen to term its polar directions, as to the inclination of its axis. Neither would produce the phenomena without the assistance of the other, as our experiment with the table will show.
Place, then, the globe at the south side of the rim of the table, with its axis inclining towards its surface, and its poles always pointing in the same general direction, not following the circuit of the orbit, and set it in motion towards the east, revolving rapidly on its axis as it moves. While directly south of the light, it would be found that the north pole would be illuminated, while no revolution on the axis would bring the south pole within the circle of the light. This is when a line drawn from the axis of the globe would cut the lamp, were the inclination brought as low as the surface of the table. Next set the globe in motion, following the rim of the table, and proceeding to the east or right hand, keeping its axis always looking in the same general direction, or in an attitude that would be parallel to a north and south line drawn through the sun, were the inclination as low as the surface of the table. This movement would be, in one sense, sideways, the circle of light gradually lessening around the north pole, and extending towards the south, as the globe proceeded east and north, diminishing the length of the days in the northern hemisphere, and increasing them in the southern. When at east, the most direct rays of the light would fall on the equator, and the light would cut the two poles, rendering the days and nights equal. As the globe moved north, the circle of light would be found to increase around the south pole, while none at all touched the north. When on the north side of the table, the northern pole of the globe would incline so far from the sun as to leave a space around it in shadow that would be of precisely the same size as had been the space of light when it was placed on the opposite side of the table. Going round the circle west, the same phenomena would be seen, until coming directly south of the lamp, the north pole would again come into light altogether, and the south equally into shadow.
Owing to this very simple but very wonderful provision of divine power and wisdom, this earth enjoys the relief of the changes in the seasons, as well as the variations in the length of the days. For one half of the year, or from equinox to equinox, from the time when the globe is at a due-west point of the table until it reaches the east, the north pole would always receive the light, in a circle around it, that would gradually increase and diminish; and for the other half, the same would be true of the other hemisphere. Of course there is a precise point on the earth where this polar illumination ceases. The shape of the illuminated part is circular; and placing the point of a pencil on the globe at the extremest spot on the circle, holding it there while the globe is turned on its axis, the lines made would just include the portions of the earth around the globe that thus receives the rays of the sun at midsummer. These lines compose what are termed the arctic and antarctic circles, with the last of which our legend has now a most serious connection. After all, we are by no means certain that we have made our meaning as obvious as we could wish, it being very difficult to explain phenomena of this nature clearly, without actually experimenting.
It is usual to say that there are six months day and six months night in the polar basins. This is true, literally, at the poles only; but, approximatively, it is true as a whole. We apprehend that few persons--none, perhaps, but those who are in habits of study--form correct notions of the extent of what may be termed the icy seas. As the polar circles are in 23° 28″, a line drawn through the south pole, for instance, commencing on one side of the earth at the antarctic circle, and extending to the other, would traverse a distance materially exceeding that between New York and Lisbon. This would make those frozen regions cover a portion of this globe that is almost as large as the whole of the Atlantic Ocean, as far south as the equator. Any one can imagine what must be the influence of frost over so vast a surface, in reproducing itself, since the presence of ice-bergs is thought to affect our climate, when many of them drift far south in summer. As power produces power, riches wealth, so does cold produce cold. Fill, then, in a certain degree, a space as large as the North Atlantic Ocean with ice in all its varieties, fixed, mountain and field, berg and floe, and one may get a tolerably accurate notion of the severity of its winters, when the sun is scarce seen above the horizon at all, and then only to shed its rays so obliquely as to be little better than a chill-looking orb of light, placed in the heavens simply to divide the day from the night.
This, then, was the region that Roswell Gardiner was so very anxious to leave; the winter he so much dreaded. Mary Pratt was before him, to say nothing of his duty to the deacon; while behind him was the vast polar ocean just described, about to be veiled in the freezing obscurity of its long and gloomy twilight, if not of absolute night. No wonder, therefore, that when he trimmed his sails that evening, to beat out of the great bay, that it was done with the earnestness with which we all perform duties of the highest import, when they are known to affect our well-being, visibly and directly.
"Keep her a good full, Mr. Hazard," said Roswell, as he was leaving the deck, to take the first sleep in which he had indulged for four-and-twenty hours; "and let her go through the water. We are behind our time, and must keep in motion. Give me a call if anything like ice appears in a serious way."