We also see, that at the same time the sun is rising at London, it is rising at the isles of Sicilly and Madagascar.

And, that at the same time when the sun sets at London, it is setting at the island of Madeira, and Cape Horn:

And, when the sun is setting at the island of Borneo, in the East Indies, it is rising at Florida, in America. And many other similar circumstances, relative to other places, are seen, as it were, by inspection.

PROBLEM XIV. To explain the situation of the earth, with respect to the sun, at the time of the winter solstice.

Rectify the globe to the extremity of the divisions for the month of December, or to 231 degrees south declination.

When it will be apparent that the whole space within the southern polar circle is in the sun's light, and enjoys continual day; whilst that of the northern polar circle is in the shade, and has continual night.

If the globe be turned round, as before, the horary index will shew that, at the several places before-mentioned, their days will be respectively equal to what their nights were at the time of the summer solstice.

It will appear, farther, that it is now sun-setting at the same time in those places in which it was sun-rising at the same time at the summer solstice ; and, on the contrary, sun-rising at the time it then appeared to set.

PROBLEM XV. To place the globe in the situation of the earth, at the times of the equinox.

The sun has no declination at the times of the equinox; consequently, there must be no elevation of the pole.

Bring the day of the month when the sun enters the first point of Aries, or day of the month when the sun enters the first point of Libra, to the plane of the broad paper circle; then the two poles of the globe will be in that plane also, and the globe will be in that position, which is called a right sphere.

For it is a right sphere when the two poles are in the plane of the broad paper circle, because then all those circles, which are parallel to the equator, will be at right angles to that plane.

If the globe be now turned from west to east, it will plainly appear, that all places upon its surface are twelve hours above the broad paper circle, and twelve hours below it; that is, the days are twelve

hours long all over the earth, and the nights are equal to the days; whence, these times are called the times of equinox.

Two of these occur in every year; the first is the autumnal, the second the vernal equinox.

At these seasons, the sun appears to rise at the same time, to all places that are on the same meridian. The sun sets, also, at the same time in all those places.

Thus, if London, and Mundford, on the Gold Coast, be brought to the strong brass meridian, the graduated side of which is, in this case, the horary index, and they be afterwards carried to the western edge of the broad paper circle, the index will shew that the sun rises at VI at both places; when they are carried to the eastern edge, the index points to VI for the time of sun-setting.

N.B. If London be not the given place, the hour index is to be set to the most elevated XII, while the place is under the graduated edge of the strong brass meridian.

The following circumstances, which usually attend the four cardinal divisions of the year, cannot be better introduced than at this place. At the time of the equinoxes, when the sun passes from one hemisphere into the other, there is almost constantly some disturbance in the weather; the winds are then generally higher: at the vernal equinox, they are, for the most part, easterly, cold, dry, and searching. The solstitial point of the summer is often distinguished by violent rains, and what we call a mid

summer flood. The winter being less rainy than the summer, nothing particularly happens at the winter solstice, but that the frosts commonly set in more severely, with some quantity of snow, upon the ground.

OF THE ARTIFICIAL OR TERRESTRIAL HORIZON.

The brass circle, which may be slipped from pole to pole on the moveable meridian, has been already described. The circumference of it is divided into eight parts, to which are affixed the initial letters of the mariner's compass.

When the centre of it is set to any particular place, the situation of any other place is seen, with respect to that place; that is, whether they be east, west, north, or south of it.

It will, therefore, represent the horizon of that. place.

We shall here use the artificial horizon, to shew why the sun, although he be always in one and the same place, appears to the inhabitants of the earth at different altitudes, and in different azimuths.

PROBLEM XVI. To exemplify the sun's altitude, as observed with an artificial horizon.

The altitude of the sun is greater or less, according as the line which goes from us to the sun is nearer to, or farther off, from our horizon.

Let the moveable circle be applied to any place,

as London, then will the horizon of London be thereby represented.

The sun is supposed, as before, to be in the zenith, that is, directly over the terrestrial globe.

If, then, from London, a line go vertically upwards, the sun will be seen at London in that line.

At sun-rising, when London is brought to the west edge of the broad paper circle, the supposed line will be parallel to the artificial horizon, and the sun will then be seen in the horizon.

As the globe is gradually turned from the west towards the east, the horizon will recede from that line which goes from London vertically upwards; so that the line in which the sun is seen gets further and further from the horizon; that is, the sun's altitude increases gradually.

When the horizon, and the line which goes from London vertically upwards, are arrived at the strong brass meridian, the sun is then at his greatest, or meridian altitude for that day, and the line and horizon are at the largest angle they can make with each other.

After this, the motion of the globe being continued, the angle between the artificial horizon and the line which goes from London vertically upwards continually decreases, until London arrives at the eastern edge of the broad paper circle; its horizon then becomes vertical again, and parallel to the line which goes vertically upwards. The sun will again appear in the horizon, and will set.

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