eleven o'clock in the morning 15 degrees west of Greenwich, and one o'clock in the afternoon 15 degrees east of Greenwich.

So the circles of longitude, thus drawn on the globe, will show the difference of time between places at different parts of the earth; they are therefore called hour circles.

CHAPTER L.

THE ANNUAL MOTION OF THE EARTH.

THE earth turns round its axis once in twenty-four hours. At the same time it is moving round the sun, and accomplishes its circuit in a year.

Imagine a spherical humming-top spinning, but, instead of resting in one spot, suppose it to move in a circle round a lamp set upon the floor. This will give some idea of the two motions of the earth. The spinning round the axis represents the diurnal, or daily motion; the circular movement round the lamp, the annual or yearly motion, the lamp being the sun. The path in which the earth moves, corresponding to the circle round the lamp, is called the orbit of the earth.

Now, if the axis of the top be vertical, one-half of the top will always be light, and the other dark, and the light will be divided from the dark by a great circle passing through the poles. As the top turns round its axis, every part will in turn be light and dark. But if the axis at one part of the orbit were inclined a little towards the lamp, then, as the top spun round, the upper pole, and a small part of the top round it, would continue light, and the lower pole and the parts round it dark, so long as the top remained at that point in the orbit. The parts also nearer the upper pole would be in the light for a longer time than they would be in

the dark, and those nearer the lower pole would be the longer time in the dark.

If the axis were always inclined in the same direction, when the top came to the point in its orbit exactly opposite to that we have just considered, the upper pole would now turn away from the lamp, and would be always dark, and the lower pole always light, and the parts which were before longer in the light would now be longer in the dark, and vice versa.* But when the top is halfway between these two points in its orbit, the axis will not incline either to or from the lamp, and the circle which separates light from dark will pass through the poles, and every part of the top will be light and dark for the same time while the top spins round at these points.

A top moving in this way will give a most exact idea of the motion of the earth.†

The course in which the earth moves, or the orbit of the earth, is nearly a circle, of which the sun is the centre; and an imaginary plane, passing through the centre of the sun and this orbit, is called the plane of the earth's orbit, as we might call the floor on which our top spun, the plane of the top's orbit round the lamp.

The axis of the earth is inclined to the plane of its orbit, just as we supposed the axis of the top to be; and is always inclined in the same direction.

When the earth is at that point in its orbit at which the axis is most inclined towards the sun, so that its

* Vice versa, two Latin words, meaning by a change of turns. The phrase is used when two things change places by turns. Thus, what was light is dark, and vice versa," means "what was light is dark, and what was dark is light"-the light and dark have changed places.

+ That this illustration may not give a wrong notion of the distance of the earth from the sun, it is proper to add that if the earth be represented by a common-sized orange, the distance from the earth to the sun, or the radius of the earth's orbit (in the same proportion), will be 1000 yards, or more than half a mile.

northern extremity points towards the sun, and the southern extremity recedes from it, the days are longer than the nights in the Northern Hemisphere, and at the parts near the North Pole it is always day. This is midsummer in the Northern Hemisphere. In the Southern Hemisphere, on the contrary, the nights are longer than the days, and in the south polar regions it is always night; it is midwinter.

When the earth is at the opposite point in its orbit, the contrary takes place; it is midwinter in the Northern Hemisphere, and midsummer in the Southern.

These two points in the earth's orbit are called, respectively, the summer and the winter Solstice. The summer solstice is on June 21st, and the winter solstice on December 21st.*

When the axis of the earth is not inclined towards or from the sun, the day and night are of equal length all over the globe.

These points in the earth are called the Vernal (or Spring) and the Autumnal Equinoxes. The Spring Equinox is on March 21st, and the Autumnal Equinox on September 21st.

As the earth moves in its orbit from the Summer to the Winter Solstices, the days in the Northern Hemisphere shorten and the nights lengthen, there being one point of time between the two when night and day are exactly equal; and as the earth moves from the Winter to the Summer Solstice, the nights shorten and the days lengthen, there being one point of time also between the two when day and night are equal.

The converse takes place in the Southern Hemisphere. It is spring in the Southern Hemisphere when it is autumn in the Northern, and it is summer in the Southern when it is winter in the Northern Hemisphere.

*Solstice means a standing or resting of the sun. The word is used because, for a few days before and after the days mentioned, the difference in their length is so small as to be imperceptible.

Christmas Day occurs in the height of summer in New Zealand and Australia.

It will be easily seen that the nearer, a place is to the poles, the longer the days are in summer, and the shorter in winter; and exactly at the poles the sun is always above the horizon during one half of the year, and always below during the other half.

In countries near the equator at some times of the year, the sun at noon is immediately overhead, or, as it is called, vertical. This makes these places very hot, because the more nearly vertical the sun is, the greater heat it gives.

Such countries are called tropical. In parts of the earth more distant from the equator, the sun is never quite vertical, but is more nearly so in summer than in winter. In northern latitudes, the sun is always in the south at noon; in southern latitudes, in the north, being higher or lower in the sky according to the time of year.

It is hotter in summer than in winter, for reasons which both depend upon the inclination of the earth's axis: first, because the days are longer; and secondly, because the rays of the sun are more nearly vertical.

Were the axis of the earth vertical, there would be no change of seasons throughout the year. The alternation of spring, summer, autumn, and winter, with the succession of fruit and flowers, and all the varied bounties of the circling year, are some of the wonderful results of what has been well called "the simple but stupendous contrivance" of the inclination of the earth's axis to the plane of its orbit;-a contrivance, like the rest of God's works, full of wisdom and goodness, and well adapted to lead the thoughtful mind to admire, love, and praise the Almighty and Allwise Creator of the Universe!

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CHAPTER LI.

THE SUNBEAM.-PART I.

SUPPOSE we were asked, Why is this grass so green, that flower so fair in its coloured raiment, this gentle air so warm and balmy, and every object around us glittering with light? We must apply to the sunbeam for an answer; and in the answer we shall find that, pervading all nature and performing the most important part in its operations, there are three distinct principles, all united in the sun-ray-Heat, Light, and Chemical power, or 66 Actinism." Could we break up these bright streams. of light which are now pouring down on hill and dale, giving joy, gladness, and life to the scene, into individual rays, and, by some process, separate one from all the rest, we should find all three in it: that is, we should find it to consist of a ray of heat, a ray of light, and a ray of chemical force, or "actinism"-each of these rays being itself made up of several others. A simple experiment will prove that these three different classes of rays coexist in a sunbeam. If, on a bright summer day, we allow the rays of the sun to pass through a hole into a dark room and then through a glass prism, we shall find on holding up a white card at a certain distance off the prism, that the ray of light is broken up into a strip of various colours, which is called "the prismatic spectrum."

This spectrum shows us that the ray of light is made up of several rays-three, as it is commonly now believed; the others being made up by the mingling of these in various ways together, and puts before us in the following order-Violet, Indigo, Blue, Green, Yellow, Orange, and Red. This streak of beautiful colours exhibits to us, then, the Light of the solar ray: how are we to detect its Heat and Actinic force? If we take & very delicate thermometer, and put it first in the violet part of the streak, then bring it gradually downwards