the fruits of the Earth, whether to be used directly by himself as articles of food, or to nourish those animals which subsequently furnish him with diet. The summer's Sun is also necessary to melt the ice, which makes the polar regions one sterile and dreary waste, unfit alike for almost every species of animal or vegetable. Were it not that the genial warmth of the summer's Sun dissolves large portions of ice, the frozen regions would gradually extend their boundary towards the equator; and those countries now called temperate, and which present fit localities for the habitations of man, would become ice-bound, and consequently uninhabitable.

But do our wants cease here? should we be content with constant summer? No; the grass would become withered, the flocks would lose the food with which they were for a while nourished, the brooks and rivulets would be dried up, pestilential diseases would ensue from the putrefaction of animal and vegetable bodies, and man himself, enervated and debilitated by excess of heat, would droop as if the vivid spark of life were sinking out. These results are prevented by the wise ordinations of Providence, by which summer is succeeded by autumn; a season which at the same time softens the descent from summer to winter, and brings to perfection those fruits which are to serve for the good of man, and other animals, during the remaining portions of the year.

Thus, then, does each season bring with it its joys and advantages; and thus are we taught to admire the exquisite mechanism and adjustment of the heavenly bodies, by which these joys and advantages are imparted to us. But how are these seasons brought about? and why is it that the Sun sheds such a vary

ing amount of cheering influence on the Earth at different seasons? The answer to this question is admirably calculated to show what wonderful effects spring from apparently small causes.

The reason for the succession

of the various seasons is to be found in the simple circumstance, that the Earth does not revolve on her axis in the same plane in which she revolves round the Sun; if those two planes coincided, we should have no variety of seasons; the same intensity of heat or of cold would be experienced at the same place, at every season of the year. This important circumstance we now proceed to explain and illustrate.

Before we enter upon that inquiry, however, it will be necessary to describe certain circles, which are supposed to be drawn on the surface of the Earth, and in doing so we must refer to the figure of the globe, page 175.

The Earth is, as before stated, nearly a perfect globe, of which the equatorial diameter is a little greater than the polar; and it has likewise been observed, that this globe, besides its revolution round the Sun in the course of a year, revolves on its own axis from west to east, once in every twenty-four hours. The two ends of this axis are the north and south poles, P and Q. The circle which surrounds the Earth, exactly midway between the two poles, is called the equator, E R. Another line crossing the equator in an oblique direction, but not given in the figure, is called the ecliptic, from the circumstance that whenever an eclipse occurs, either solar or lunar, the Sun, the Moon, and the Earth are all quite, or very nearly, in this plane. The ecliptic is a circle which, like the equator, goes round the Earth, and represents the apparent p th of the Sun

in the course of a year; that is, if a body were held exactly in a right line between the centre of the Sun and the centre of the Earth, the shadow of that body would travel along the ecliptic. These two circles, the equator and the ecliptic, are called greater circles, because their planes pass through the centre of the Earth, and divide the Earth into equal parts; whereas, all other circles are denominated less circles, because their planes divide the Earth into two unequal parts. There are also two circles which surround the Earth, parallel to the equator, and at a distance from it equal to about 234. These are called tropics, from a Greek word which implies turning, as the Sun, when it has arrived at those parts, begins then to turn away again northward or southward, as the case may be; the circle which is nearer to the north pole being the tropic of Cancer, and that which is nearer to the south pole being the tropic of Capricorn. The position of these two circles is determined by the greatest distance which the ecliptic recedes from the equator; and they receive their name from the circumstance of the Sun being at the time in the constellation which gives the name to the tropic. There are also two small circles which surround the poles, at a distance from the poles exactly equal to the distance of the tropics from the equator. The arctic circle is that which surrounds the north pole, at a distance of 231°; the antarctic circle is that which surrounds the south pole. The term antarctic signifies opposite to the arctic, which latter term is derived from the Greek for a bear: the little bear is a constellation over the north pole of the Earth.

Now, in the subjoined diagram, we observe an oval line, which the Earth describes annually about the Sun

at o, or which the Sun appears to describe round the Earth. The Earth moves round this orbit in a year, and we have the position of the Earth in that orbit at four different periods of the year, about three months asunder. It will be seen that the axis of the Earth N S is inclined towards one side, and that it remains parallel throughout the whole of the Earth's revolution; that is, that there is exactly the same obliquity, or sloping direction, at all seasons of the year; this causes the equator to form with the ecliptic an angle of about 231°.

Let us now suppose the time of the year to be the 21st of March, and let us trace the progress of the Earth in her annual orbit. She is then at the upper part of the figure in the position which affords the northern spring, and the Sun is at exactly the same distance from both poles; that is, if the centres of the Sun and the Earth were connected by a right line, this line would pass through the equator E Q, and

would therefore be equidistant from the two poles. Now it is not difficult to see that in this case the Sun imparts just as much heat to one pole as to the other, affecting either hemisphere alike. At about the 23rd of June the Earth is at the left, and gives the northern summer. Here the Earth is actually further from the Sun than it was three months before: but the excess of heat at this latter period is due to a circumstance altogether separate from the actual distance of the Sun. The obliquity of the axis of the Earth is the important circumstance to which we have to keep our attention. We perceive that in the northern summer, the north pole is nearer to the Sun than the south pole: hence a greater amount of heat is received from the Sun by the northern part of the Earth than by the southern at this period. This is the sole origin of the results of the seasons, summer and winter: the large amount of heat received by the northern hemisphere, in comparison with that received by the southern, produces the season' of summer to the former, and winter to the latter. at this season we mark the boundary line between the enlightened and the dark hemispheres of the Earth, it will be seen that this line will just graze the polar circles: it will likewise be seen, that if we draw a line through the centres of the Sun and Earth, that line will pass through the tropic of Cancer.

If

We next find the Earth in the lower position which she attains about the 21st of September, when the northern autumn is felt. We observe that the Earth's position is such as it was in the spring, when the two poles were exactly equidistant from the Sun. Here then we have the season of autumn, in which the direct light and

heat of the Sun are equal to what they are in spring.