other 61 yards, should you at once be able to distinguish the greater from the less?

Charles. I think not.

Tutor. Just in the same manner does the distance of the centre from the surface of the earth vanish in comparison of its distance from the moon.

James. We must not, however, forget the succession of day and night.

Tutor. Well then; if the sun be supposed at z, it will illuminate by its rays all that part of the earth that is above the horizon Ho: to the inhabitants at G, its western boundary, it will appear just rising; to those situated at R, it will be noon; and to those in the eastern part of the horizon c, it will be setting.

Charles. I see clearly why it should be noon to those who live at R, because the sun is just over their heads, but it is not so evident, why the sun must appear rising and setting to those who are at G and c.

Tutor. You are satisfied that a spectator cannot, from any place, observe more than a semicircle of the heavens at any one time; now what part of the heavens will the spectator at G observe?

James.

ZON.

He will see the concave hemisphere

Tutor. The boundary to his view will be N and z, will it not?

Charles. Yes; and consequently the sun at z, will to him be just coming into sight.

Tutor. Then, by the rotation of the earth, the spectator at G will in a few hours come to R, when, to him, it will be noon; and those who live at R, will have descended to c; now what part of the heavens will they see in this situation?

James. The concave hemisphere N Hz, and z being the boundary of their view one way, the sun will to them be setting.

Tutor. Just so. After which they will be turned away from the sun, and consequently it will be night to them till they come again to G. Thus, by this simple motion of the earth on its axis, every part of it is, by turns, enlightened and warmed by the cheering beams of the sun. Charles. Does this motion of the earth account also for the apparent motion of the fixed stars?

Tutor. It is owing to the revolution of the earth round its axis, that we imagine the whole starry firmament revolves about the earth in 24 hours.

James. If the heavens appear to turn on an axis, must there not be two points, namely, the extremities of that imaginary axis, which always keep their position?

Tutor. Yes, we must be understood to except the two celestial poles which are opposite to the poles of the earth, consequently each fixed star

appears to describe a greater or a less circle round these, according as it is more or less remote from those celestial poles.

Charles. When we turn from that hemisphere in which the sun is placed, we immediately gain sight of the other in which the stars are situated. Tutor. Every part of the heaven is decorated with these glorious bodies: and

Night opes the noblest scenes, and sheds an awe,
Which gives those venerable scenes full weight,
And deep reception in th' intender'd heart.
This gorgeous apparatus! This display!
This ostentation of creative power!
This theatre! what eye can take it in ?
By what divine enchantment was it rais'd
For minds of the first magnitude to launch
In endless speculation, and adore?

One sun by day, by night ten thousand shine:
And light us deep into the Deity;

How boundless in magnificence and might!

YOUNG.

James. If every part of the heavens be thus adorned, why do we not see the stars in the day as well as the night?

Tutor. Because in the day time, the sun's rays are so powerful, as to render those coming from the fixed stars invisible. But if you ever happen to go down into any very deep mine, or coal-pit, where the rays of the sun cannot reach the eye, and it be a clear day, you may by looking up to the heavens, see the stars at noon as well as in the night.

Charles. If the earth always revolve on its axis in 24 hours, why does the length of the days and nights differ in different seasons of the year?

Tutor. This depends on other causes connected with the earth's annual journey round the sun, upon which we will converse the next time we meet.

CONVERSATION XXX.

Of the Annual Motion of the Earth.

Tutor. Besides the diurnal motion of the earth by which the succession of day and night is produced; it has another, called its annual motion, which is the journey it performs round the sun in 365 days, 5 hours, 48 minutes, and 49 seconds.

Charles. Are the different seasons to be accounted for by this motion of the earth?

Tutor. Yes, it is the cause of the different lengths of the days and nights, and consequently of the different seasons, viz. Spring, Summer, Autumn, and Winter:

It shifts the seasons, months, and days,
The short-liv'd offspring of revolving time;
By turns they die, by turns are born.
Now cheerful Spring the circle leads
And strews with flow'rs the smiling meads;
Gay Summer next, whom russet robes adorn,
And waving fields of yellow corn;

Then Autumn, who with lavish stores the lap of Na-
ture spreads;

Decrepit Winter, laggard in the dance
(Like feeble age opprest with pain,)
A heavy season does maintain,

With driving snows and winds and rain
Till Spring recruited to advance,
The various year rolls round again.

HUGHES.

James. How is it known that the earth makes this annual journey round the sun?

Tutor. I told you yesterday, that through the shaft of a very deep mine, the stars are visible in the day as well as in the night; they are also visible in the day time, by means of a telescope properly fitted up for the purpose; by this method, the sun and stars are visible at the same time. Now if the sun be seen in a line with a fixed star, to-day at any particular hour, it will, in a few weeks, by the motion of the earth, be found considerably to the east of him and if the observations be continued through the year, we shall be able to trace him round the heavens to the same fixed star from which we set out; consequently, the sun must have made a journey round the earth in that time; or the earth round him.