If you stand opposite to a fire which is about half your height from the floor, it is obvious that the middle part of your person would receive the greatest amount of heat. And if this fire were in the middle of a large room or hall, so that you could walk round it, always keeping the same distance from it, and with your face towards it, would there be any change as to the parts of your person which would receive the greatest amount of heat? But, if in going round it in a circle, you keep turning round and round at the same time, what would be the effect? Yes; you are right, the middle part of your person would receive the greatest amount of light and heat, and the upper and lower parts the least. If you could move round it in a slanting direction, so that the upper part of your person would be inclined towards the fire, and the lower part declined from it, what would be the result? You are right; the upper part of your person would receive a greater, and the lower a less degree of heat; and this you will see clearly if you carry, in this position, a small figure, as a child's doll, round a candle placed in the middle of a circular table. And if in carrying the little figure round as before, you make the upper part of it decline from the candle, and the lower part incline to it, the result will be the reverse, that is, the upper part of it will receive less of the light and heat than the lower.

Now, if the earth moved round the sun in any of the ways we have described, there would be NO SEASONS; for the same parts of its surface would always receive the same amount of light and heat. And how does the earth move round the sun? Now mark the answer to this question, for it contains the whole doctrine of the seasons-THE AXIS OF THE EARTH IS INCLINED TO THE PLANE OF ITS ORBIT, AND IT MOVES ROUND THE SUN IN A DIRECTION PARALLEL TO ITSELF. And this is the only way it could move, for it has no power to keep changing the direction of its axis, as you kept turning and twisting the small figure in carrying it round the candle, so that its head pointed in every direction--north, south, east, and west.

Now, carry the small figure round the candle in a direction parallel to itself, and observe the different effects. If you stand on the south side of the table, and incline the upper part of the figure towards the candle, its head will point northward of the point of the ceiling which is directly above the candle. This represents the position of the earth with regard to the sun at MIDSUMMER. When you carry it a quarter way round, with its head still pointing in the same direction, that is, northward, you will observe that neither the upper, nor the lower part of the figure is inclined to, or declined from the candle; and that consequently neither of them receives a greater amount of light and heat than the other. In this position, the middle of the figure is directly opposite the middle of the flame of the candle; and that part of its surface is consequently the warmest. This is an illustration of the AUTUMNAL EQUINOX.

Now carry the small figure round another quarter of the circle

keeping its head in the same direction, that is, northward, and you will observe that the upper part of it is now declined from, and the lower part inclined to the candle. This represents the position of the earth with regard to the sun in the WINTER of our year. And if you carry it round a third quarter of the circle, with its head pointing in the same direction as before, that is, northward, you will see that neither the upper nor the lower part of it inclines to, or declines from the candle. This represents the VERNAL EQUINOX. Now carry it round the remaining quarter of the circle as before, and you will observe that the upper part of it is again inclined towards the candle, and the lower part declined from it; or, in other words, we shall have a representation of our SUMMER again.

And you will also observe that, in carrying the small figure round the candle, you made it move in the same PLANE; that is, you neither raised it nor lowered it, so as to bring at one time the parts above the middle, and at another time, the parts below it, directly opposite the middle of the flame of the candle. In this way, the earth, neither rising nor sinking in its course (which it could not de presents at one period, that part of its surface which is 23 degrees above the equator to the direct rays of the sun; at another, the equator; at another, that part of its surface which is 23 degrees below the equator; at another, the equator again; and so on.]

In the following diagram, the position of the earth with regard to the sun at midsummer, midwinter, and the equinoxes, is represented.

B

A

D

At A, the northern half of the axis is inclined to the sun, and the northern hemisphere, in consequence, enjoys much

more of his rays than the southern. In this position of the earth, the sun shines perpendicularly over the TROPIC OF CANCER, and consequently 23 degrees over and beyond the north pole; for as the earth is a globe, the sun shines over the one half of it, or in other words, over ninety degrees in every direction, from the point over which his rays are perpendicular. In this case, the entire of the north FRIGID ZONE will be within the illuminated hemisphere, and it will be constant day there while the earth remains in this position with regard to the sun. It is obvious, too, that in this case the rays of the sun will fall short of the south pole by 23 degrees, and that, consequently, the entire south FRIGID ZONE will be deprived of his light while the earth continues in this position with regard to the sun.

Suppose the earth to have moved to B, and observe that its axis is neither inclined to, nor declined from the sun. In this case, the sun is perpendicular to the equator, and consequently shines ninety degrees above and below it, or, in other words, from pole to pole. In this case too it is obvious, that the days and nights are equal all over the world; for not only the equator, but all the parallels of latitude are bisected or cut into two equal parts by the CIRCLE OF ILLUMINATION. By the circle of illumination is meant the circle which divides the hemisphere presented to the sun, from the hemisphere which is deprived of his light. And as this circle divides the globe into two equal parts, it is a GREAT CIRCLE ;* and as all great circles bisect each other, it

a A GREAT circle of a globe or sphere is one which would divide it into two equal parts or HEMISPHERES; and it is so called because it is evident that no greater could be drawn upon it. The equator is a great circle; and so is each of the meridians-and so indeed are all circles that would divide the globe into two equal parts. And it is also evident that the PLANES of all great circles must pass through the centre of the earth. To show this clearly, cut a round apple through the middle from side to side, and this will illustrate the great circle of the equator, and its plane as passing through the centre. Take another, and cut it from top to bottom through the middle, and this will show that a meridian is also a great circle, and that its plane passes through the centre. Take a third apple, and from any point of its surface, cut it through the middle to the opposite point, and you will have a representation of another great circle, and its plane passing through the centre. Now, take a fourth apple and cut it, in a circular direction, into two unequal parts, and you will see that its plane does not pass through the centre. Such circles are called SMALL CIRCLES. In fact, even children know the difference between great and small circles; for

in every situation of the earth, divides the equator into two equal parts. It is this circumstance which causes the days and nights to be of equal length at the equator throughout the year. One half of it is within the enlightened, and the other half within the darkened hemisphere; and as the entire circle turns round in twenty-four hours, it is evident, that each half of it will turn round in twelve; or in other words, the days and nights will be of equal length.

The same explanation applies to all the circles parallel to the equator, or, as they are usually called, parallels of latitude. When the earth is in the position now described, they are all bisected, or divided into two equal parts by the circle of illumination; and the days and nights are consequently equal all over the world. But when the sun is above or below the equator, that is, north or south of it, all the parallels of latitude are unequally divided by the circle of illumination, and the days and nights are consequently of unequal length. When the sun is north of the equator, more than half of each of the parallels of latitude in the northern hemisphere is within the circle of illumination, and the days are consequently longer than the nights; and when the sun is south of the equator, the contrary is evidently the

case.

In explaining to the pupils what is meant by the circle of illumination, the teacher should not trust entirely to the diagram. He will give them a clearer conception of it by holding a small globe before a candle in different positions, and by calling upon them, at every change, to point out its boundary and the direction of its plane, which, as it is a great circle, always passes through the centre of the earth. If the north pole or axis of a small globe, for example, is held opposite the candle in a straight line with the centre of the light, it will be evident that the entire northern hemisphere would be within, and the entire southern hemisphere without the circle of illumination; and that if the earth turned round in this way before the sun, it would be perpetual day in the one hemisphere, and perpetual night in the other. In this case it will be evident, that the boundary between the enlightened and shaded hemispheres, or in other words, the circle of

if you promise a child the half of an orange or an apple, he will be on the watch to see that you are cutting it in the direction of a great circle, that is, fairly through the middle.

illumination, will exactly coincide with the equator, and consequently that its plane will pass through the centre of the globe perpendicular, or at right angles to its axis.

In this case, it is obvious that the plane of the circle of illumination would be perpendicular to a line drawn from the centre of the sun to the centre of the earth, to which we suppose the sun's rays to be parallel; for in this position the axis represents that line; and it may be easily shown that it is always in every situation of the earth with regard to the sun.

So,

If the pupils get a clear idea of what is meant by the circle of illumination, keeping in mind that the earth, in moving round the sun, has its axis inclined to its orbit at an angle of 66 degrees, and that it always points to the same part of the heavens, they will feel no difficulty in comprehending the causes of the seasons, or in determining the length and general temperature of the days, in every part of the earth, throughout the year.

b

For in this case, the plane of the circle of illumination coincides with the plane of the equator which is evidently at right angles to the axis of the earth.

b If you move round the edge of a circular table in the middle of a room, a line from the top of your head will appear to describe a corresponding and equal circle on the ceiling-and yet the north pole of the earth, though it describes, in the course of a year, a circle of 190 millions of miles in diameter, always points to the polar star! This arises from the amazing distance of the fixed stars, which causes, not only the earth, but the entire orbit in which it moves to appear as a mere point in comparison. The following illustration will make this clear: If the circle formed on the floor by your moving round the table be six feet in diameter, the corresponding circle described on the ceiling will also be six feet in diameter, and its centre will, consequently, be three feet from every point of its circumference. Now, in walking round the circle on the floor your head will evidently point to the circumference of the circle on the ceiling, and not to the centre of it. But if the circle were painted black, so as to make it more distinctly seen, and if the ceiling on which it is described could be perpendicularly raised to an immense height, what effect would this have as to its appearance? You are right; it would appear to us to be much smaller; perhaps not larger than the crown of a man's hat. And if it were raised higher and higher, it would gradually diminish to the size of a black wafer; and finally, this circle, which we know to be six feet in diameter, would appear to us to be no larger than the head of a black pin. Now, if you walk round the circle as before, a line from the top of your head would, in appearance, always point to this speck, though you know it would really describe a circle six feet in diameter This may also be illustrated by drawing upon an elevation three or four parallel lines, ten or fifteen feet from each other. If we look

C