of these nodes, does it certainly produce an eclipse?

Yes, even more frequently than that, because the body of the moon is larger than these precise points but I will speak of that after I have prepared the way, by explaining to you the cause of an eclipse of the sun, and of the moon.

How kind you are, to answer my wishes rather than my questions' I wish to be wise, but ask questions which impede my progress.

Not exactly so; but to proceed. An eclipse of the sun, is occasioned by the dark body of the moon, passing between the earth, and the sun, or by the shadow of the moon falling on the earth, at the place where the observer is situated: hence all the eclipses of the sun happen at the time of the new moon.

The last drawing answered the end so well, may I beg for another?

There is another? now let S, represent the sun, m, the moon; between the earth and the sun, a E Gb, a portion of the moon's orbit; e and 1 two places on the surface of the earth. I thank you for this; but what do the parts, in shadow, represent?

That, marked 7, is the moon's shadow, and is called the umbra, and the light part the penumbra: now it is evident, that, if a spectator is situated in that part of the earth, where the umbra falls, there will be a total eclipse of the

stin at that place; at 8 and 9 in the penumbra there will be a partial eclipse, and beyond the penumbra there will be no eclipse at all.

Is it then the fact, that there may be a total eclipse of the sun to the inhabitants of Africa, while to those at the north pole there is no eclipse at all?

Exactly so; for the sun is not deprived of his light during a solar eclipse; and, as the moon's shadow in its passage over the earth from west to east only covers a small part of the earth's enlightened hemisphere at once, your inference is evidently correct.

How much of the earth's enlightened hemisphere, does the shadow cover?

At the moon's least distance from the earth, her dark shadow, covers a circumference of but one hundred and seventy miles, if the time of the eclipse be about noon: but much more if the time be in the morning or evening, because the shadow falls obliquely on the earth. To all, who are within this circumference, the sun will appear totally eclipsed, for about five minutes; but to no place beyond it, although the Gun will be partially eclipsed for several hundred miles round.

But, of course, this spot or circumference <f darkness journeys on, as the moon which tauses it, advances ?

Certainly; and it perpetuates the darkness

for one hundred and seventy miles during the movement, gradually changing its situation all the time; for the shadow passes like a cloud over the earth, at the rate of two thousand miles an hour.

You said just now, that, at the moon's least distance from the earth, her shadow would pass over one hundred and seventy miles; but I wish to know, how this could take place, if an eclipse were to happen, when the earth is so far from the moon, that the lines 8—10, 9—11, would cross each other, before they come to the earth?

A person, situated on the earth, in a direct line between the centres of the sun and moon, would see a ring of light round the dark body of the moon, called an annular eclipse: when this happens, there is not a total eclipse any where, because the moon's umbra does not reach the earth.

How long does such an eclipse usually last?

An annular eclipse does not continue longer than twelve minutes, nor a total more than seven minutes. The duration of an eclipse of the sun can never exceed two hours.

Will you explain to me the cause of an eclipse of the moon?

This arises from her entering the earth's shadow, and consequently it must happen, when she is in opposition to the sun, that is, at the