the atmosphere, which occasions the refraction of the sun's rays, and thus imparts light some days sooner, and occasions it to continue some days longer than it would otherwise remain. Again, the sun never goes more than twentythree degrees and a half below the horizon of the pole, so that the inhabitants have very little dark night; since, with them as well as with us, there is twilight till the sun is eighteen degrees below the horizon.

The Aurora Borealis, which some naturalists suppose to be a magnetic substance, which, gathering and thickening towards the north, sheds a certain light at a distance, is not without its influence in cheering their gloom.

But still, from the subject of our conversation, I apprehend you have left the most important mitigation, of the situation of the polar inhabitants, for me to name.

Well, then, let me hear what it is.

It is their enjoying so much light from the moon; but though I conclude this to be the fact, I do not understand why the absence of the sun secures the visits of the moon.

This I will endeavour to explain to you.—

"The full moon, being always opposite to the sun, can never be seen while the sun is above the horizon, except when the moon falls in the northern half of her orbit; for, whenever any point of the ecliptic rises, the opposite point sets. There

fore, as the sun is above the horizon of the north pole, from the 20th of March, till the 23d of September, it is plain that the moon, when full, being opposite to the sun, must be below the horizon during that half of the year; but when the sun is in the southern half of the ecliptic, he never rises to the north pole; during which half of the year, every full moon happens in some part of the northern half of the ecliptic, which never sets. Consequently, as the polar inhabitants never see the full moon in summer, they have her always in the winter, before, at, and after the full, shining for fourteen of our days and nights; and when the sun is at his greatest depression below the horizon, being then in Capricorn, the moon is at her first quarter in Aries, full in Cancer, at her third quarter in Libra; and as the beginning of Aries is the rising point of the ecliptic, Cancer the highest, and Libra the setting point; the moon rises at her first quarter in Aries, is most elevated above the horizon, and full in Cancer, and sets at the beginning of Libra in her third quarter, having continued visible for fourteen diurnal rotations of the earth. Thus the poles are supplied, one half of the winter-time, with constant moonlight in the sun's absence, and only lose sight of the moon from her third to her first quarter, while she gives but very little light, and could be but of little, and sometimes of no service to them."

But still, lest the idea which I wish to communicate, should be imperfectly conveyed, I will introduce a few additional obervations in the form of a memorandum at the close of our conversation, and this shall be illustrated by a drawing. But I wish to point out to you in the phenomenon which attends the harvest-moon, the wisdom and beneficence of the Deity, who so orders the course of this luminary, that it bestows more or less light on all the earth, as the several circumstances of its inhabitants require, and as the varying seasons render it serviceable and necessary.

Pray do so, my dear mamma.

Many persons take it for granted, that the moon rises about fifty minutes later every day than on the day that preceded; but this is the case only at the equator, where there is no variety of seasons, where the weather seldom changes, and even then but at stated times, so that moonlight is not necessary for gathering in the produce of the earth. But, at consider able distances from the equator, where the weather, and the seasons, are more uncertain, the autumnal full moon rises very soon after sunset for several evenings together. At the polar circles, where the mild season is of very short duration, the autumnal full moon rises at

* Memorandum IV.

sunset, from the first to the third quarter; and at the poles, where the sun is for half a year absent, the winter full moons shine constantly without setting, from the first to the third quarter.

I think I know the reason of this.

I presume you understand that all these phenomena are owing to the different angles made by the horizon and different parts of the moon's orbit; and that the moon can be full, but once or twice in a year in those parts of her orbit which rise with the least angles, and can never be full, but when she is opposite to the sun; and since the sun is never in Virgo and Libra, excepting in our autumnal months, it is plain that the moon is never full in the opposite signs Pisces and Aries but at that season; and, therefore, we can only have two full moons in the year, which rise so near the time of sunset for a week together, as the horizontal moon and its immediate successor.

the

But is not the moon in Aries twelve times in year?

Yes.

Why then do we not observe this remarkable rising of the moon except at the time of harvest? The reasons are, in winter these signs rise at noon, when the superior light of the sun prevents her being regarded. In the spring, the