nus in b, and the earth, farther from that luminary than either of them, makes its circuit in t. James. How is this known?

Tutor. By observation; for by attentively watching the progress of these bodies, it is found that they are continually changing their places among the fixed stars, and that they are never seen in opposition to the sun; that is, they are never seen in the western side of the heavens in the morning when he appears in the east; nor in the eastern part of the heavens in the evening when the sun appears in the west.

Charles. Then they may be considered as attendants upon the sun?

Tutor. They may: Mercury is never seen from the earth at a greater distance from the sun, than about twenty-eight degrees, or about as far as the moon appears to be from the sun on the second day after its change; hence it is that we so seldom see him; and when we do, it is for so short a time, and always in twilight. that sufficient observations have not been made to ascertain whether he has a diurnal motion on his axis.

James. Would you then conclude that he has such a motion?

Tutor. I think we ought; because it is known to exist in all those planets upon which observations of sufficient extent have been made, and therefore we may surely infer, without much chance of error, that it belongs also to Mercury,

and the Herschel, the former from its vicinity to the sun, and the latter from its great distance from that body, having at present eluded the investigation of the most indefatigable astrono

mers.

Charles. At what distance is Mercury from the sun?

Tutor. He revolves round that body at about thirty-seven millions of miles distance, in eighty-eight days nearly; and therefore you can now tell me how many miles he travels in an hour.

James. I can; for supposing his orbit circular, I must multiply the 37 millions by 6,* which will give 222 millions of miles for the length of his orbit; this I shall divide by 88, the number of days he takes in performing his journey, and the quotient resulting from this, must be divided by 24, for the number of hours in a day; and by these operations, I find that Mercury travels at the rate of more than 105,000 miles in an hour.

Charles. How large is Mercury?

Tutor. He is the smallest of all the planets. His diameter is something more than 3200 miles in length.

James. His situation being so much nearer to the sun than ours, he must enjoy a considerably greater share of its heat and light.

Tutor. So much so, as would indeed infallį

* See p. 160.

bly burn every thing belonging to the earth to atoms, were she similarly situated. The heat of the sun at Mercury, must be seven times greater than our summer heat:

-Mercury the first

Near ordering on the day, with speedy wheel
Flies swiftest on, inflaming where he comes,
With seven-fold splendour, all the azure road.

MALLET'S EXCURSION.

Charles. And do you imagine that, thus circumstanced, this planet can be inhabited?

Tutor. Not by such beings as we are: you and I could not long exist at the bottom of the sea; yet the sea is the habitation of millions of living creatures; why then may there not be inhabitants in Mercury, fitted for the enjoyment of the situation which that planet is calculated to afford? If there be not, we must be at a loss to know why such a body was formed; certainly it could not be intended for our benefit, for it is rarely even seen by us:

Ask for what end the heavenly bodies shine?

Earth for whose use ? Pride answers, ""Tis for mine: -suns to light me rise,

My footstool earth, my canopy the skies."

POPE.

But do these worlds display their beams, or guide
Their orbs, to serve thy use, to please thy pride?
Thyself but dust, thy stature but a span,
A moment thy duration; foolish man!
As well may the minutest emmet say,
That Caucasus was raised to pave his way:
The snail, that Lebanon's extended wood

Was destined only for his walk and food:
The vilest cockle, gaping on the coast
That rounds the ample seas, as well may boast,
The craggy rock projects above the sky,

That he in safety at its foot may lie;

And the whole ocean's confluent waters swell,

Only to quench his thirst, or move and blanch his shell.

CONVERSATION XL.

PRIOR.

Of Venus.

Tutor. We now proceed to Venus, the second planet in the order of the solar system, but by far the most beautiful of them all:

Fairest of stars, last in the train of night,
If better thou belong not to the dawn,

Sure pledge of day, that crown'st the smiling morn
With thy bright circlet, praise him in thy sphere,
While day arises, that sweet hour of prime.

MILTON.

James. How far is Venus from the sun? Tutor. That planet is sixty-eight millions of miles from the sun, and she finishes her journey in 2244 days, consequently she must travel at the rate of 75,000 miles in an hour.

Charles. Venus is larger than Mercury, I dare say?

Tutor. Yes, she is nearly as large as the earth, which she resembles also in other respects,

her diameter being about 7700 miles in length, and she has a rotation about her axis in 23 hours and 20 minutes. The quantity of light and heat which she enjoys from the sun, must be double that which is experienced by the inhabitants of this globe.

James. Is there also a difference in her seasons, as there is here?

Tutor. Yes, in a much more considerable degree. The axis of Venus inclines about 75 degrees, but that of the earth inclines only 230 degrees, and as the variety of the seasons in every planet depends on the degree of the inclination of its axis, it is evident that the seasons must vary more with Venus than with us.

Charles. Venus appears to us larger sometimes than at others.

Tutor. She does; and this, with other particu lars, I will explain by means of a figure. Suppose s (Plate VII. Fig 17.) to be the sun, T the earth in her orbit, and a, b, c, d, e, f, Venus in her's now it is evident that when Venus is at a, between the sun and earth, she would, if visible, appear much larger than when she is at din opposition.

James. That is because she is so much nearer in the former case than in the latter, being in the situation a, but 27 millions of miles from the earth T, but at d, she is 163 millions of miles off. Tutor. Now as Venus passes from a, through b, c, to d, she may be observed by means of a