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.

བྱས་-

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

Charles. And the sun being a million of times larger than the earth, you will say that it is more natural, that the smaller body should go round the larger, than the reverse.

Tutor. That is a proper argument; but it may be stated in a much stronger manner. The sun and earth mutually attract one another, and since they are in equilibrio by this attraction, you know, their momenta must be equal,* therefore the earth being the smaller body, must make out by its motion what it wants in the quantity of its matter, and, of course, it must be that which performs the journey.

James. But if you refer to the principle of the lever, to explain the mutual attraction of the sun and earth, it is evident that both bodies must turn round some point as a common centre. Tutor. They do; and that is the common centre of gravity of the two bodies. Now this point between the earth and sun is within the surface of the latter body.

Charles. I understand how this is; because the centre of gravity between any two bodies, must be as much nearer to the centre of the larger body than the smaller, as the former contains a greater quantity of matter than the lat

ter.

Tutor. You are right: but you will not conclude that, because the sun is a million of times

* See Conversation XIV. p. 73.

VOL. I.P

larger than the earth, therefore it contains a quantity of matter, a million of times greater than that contained in the earth.

James. Is it then known, that the earth is composed of matter more dense than that which composes the body of the sun?

Tutor. The earth is composed of matter four times denser than that of the sun; and hence the quantity of matter in the sun is between two and three hundred thousand times greater than that which is contained in the earth.

Charles. Then for the momenta of these two bodies to be equal, the velocity of the earth must be between two and three hundred thousand times greater than that of the sun.

Tutor. It must: and to effect this, the centre of gravity between the sun and earth, must be as much nearer to the centre of the sun, than it is to the centre of the earth, as the former body contains a greater quantity of matter than the latter and hence it is found to be several thousand miles within the surface of the sun.

James. I now clearly perceive, that since one of these bodies revolves about the other in the space of a year, and that they both move round their common centre of gravity, that it must, of necessity, be the earth which revolves about the sun, and not the sun round the earth.

Tutor. Your inference is just. To suppose that the sun moves round the earth, is as absurd as to maintain, that a mill-stone could be made to move round a pebble.