longer, the sun rising and setting obliquely. The greater the latitude of the place, the longer is the duration of the twilight; so that all those who are in 49 degrees of latitude, have, in the summer, near the solstice, their atmosphere enlightened the whole night, the twilight lasting till sun-rising.

In a parallel sphere, the twilight lasts for several months; so that the inhabitants of this position have either direct or reflex light of the sun nearly all the year, as will plainly appear by the globe:

OF THE DIURNAL MOTION OF THE EARTH, AND THE PROBLEMS DEPENDING ON THAT MOTION.

As the daily motion of the earth about its axis, and the phenomena dependant on it, are some of the most essential points which a beginner ought to have in view, we shall now endeavour to explain them by the globes; and here I think the advantage of globes, mounted in my father's manner, over those generally used, will be very evident.

I have already observed, that in globes mounted in our manner, the motion of the terrestrial globe about its axis, represents the diurnal motion of the earth, and that the horary index will point out upon the equator the 24 hours of one diurnal rotation, or any part of that time.

I shall now consider the broad paper circle as the plane which distinguishes light from darkness; that is, the enlightened half of the earth's surface, from that which is not enlightened.

For when the sun shines upon a globe, he shines only upon half of it; that is, one half of the globe's surface is enlightened by him, the other not.

That the enlightened half may be that half which is above the broad paper circle, we must imagine the şun to be in our zenith.

Or, let a sun be painted on the ceiling over the terrestrial globe, the diameter of the picture equal to the diameter of the globe.

Then, all those places that are above the broad paper circle, will be in the sun's light; that is, it will be in all those places.

And all places that are below this circle, will be out of the sun's light; that is, in all those places it will be night.

When any place on the earth's surface comes to the edge of the broad paper circle, passing out of the shade into the light, the sun will appear rising at that place.

And when a place is at the edge of the broad paper circle, going out of the light into the shade, the sun will appear, at that place, to be setting.

When we view the globe in this position, we at once see the situation of all places in the illuminated hemisphere, whose inhabitants enjoy the light of the day. One edge of the broad paper circle shews at what place the sun appears rising at the same time; and the opposite edge shews at what place the sun is setting at the same time.

The horary index shews how long a place is moving from one edge to the other; that is, how long the

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TO RECTIFY THE TERRESTRIAL GLOBE.

day or night is at that place; and, consequently, when the globe is thus situated, you readily discover the time of the sun's rising and setting, on any given day, in any place.

TO RECTIFY THE TERRESTRIAL GLOBE.

To rectify the terrestrial globe, is to place it in the same position in which our earth stands to the sun, at all, or at any given times.

That half of the earth's surface, which is enlightened by the sun, is not always the same; it differs according as the sun's declination differs.

To rectify, then, the terrestrial globe, is to bring it into such a position, as that the enlightened half of the earth's surface may be all above the broad paper circle.

On the back side of the strong brass meridian, and on each side of the north pole, the months, and days of the month, are graduated in two concentric spaces, agreeable to the declination of the sun.

Bring the day of the month that is graduated on the back side of the strong brass meridian, to coincide with the broad paper circle, and the globe is rectified.

Thus, set the first of May to coincide with the broad paper circle, and that half of the earth's surface, which is enlightened at any time upon that day,

will be all at once above the said circle.

If the horary index be set to XII. when any par

ticular place is brought under the strong brass meridian, it will shew the precise time of sun-rising and sun-setting at that place, according as that place is brought to the eastern or western edge of the broad paper circle.

It will also shew how long any place is in moving from the east to the west side of the illuminated disc, and thence the length of day and night.

It will also point out the length of the twilight, by shewing the time in which the place is passing from the twilight circle to the edge of the broad paper circle on the western side; or, from the edge of this circle on the eastern side, to the twilight wire, and thus determine the length of the whole artificial day.

N. B. The twilight wire is placed at 18 degrees from the broad paper circle.

I shall now proceed to exemplify, upon the globes, these particulars, at three different seasons of the year, viz, the summer solstice, the winter solstice, and the time or times of the equinoxes,

PROBLEM XIII. To place the globe in the same situation, with respect to the sun, as our earth is in at the time of the summer solstice.

Rectify the globe to the extremity of the divisions for the month of June, or 23 degrees north declination; that is, bring these divisions on the strong brass meridian, to coincide with the plane of the broad paper circle.

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Then, that part of the earth's surface, which is within the northern polar circle, will be above the broad paper circle, and will be in the light, and the inhabitants thereof will have no night.

But all that space, which is contained within the southern polar circle, will continue in the shade; that is, it will there be continual night.

In this position of the globe, the pupil will observe how much the diurnal arches of the parallels of latitude decrease, as they are more and more distant from the elevated pole.

If any place be brought under the strong brass meridian, and the horary index is set to that XII which is most elevated, and the place be afterwards brought to the western side of the broad paper circle, the hour index will shew the time of sun rising; and when the place is moved to the eastern edge, the index points to the time of sun-setting.

The length of the day is obtained by the time shewn by the horary index, while the place on the globe moves from the west to the east side of the broad paper circle.

Thus, it will be found, that at London the sun rises about 15 minutes before IV in the morning, and sets about 15 minutes after VIII at night.

At the following places, it will be nearly at the times expressed in the table.