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PROBLEM XVII. Of the sun's meridian altitude, at three different seasons.

Rectify the globe to the time of the winter solstice, by Problem xiv, and place the centre of the visible horizon on London.

When London is at the graduated edge of the strong brass meridian, the line which goes vertically upwards, makes an angle of about 15 degrees; this is the sun's meridian altitude at that season to the inhabitants of London.

If the globe be rectified to the times of equinox, by Problem xv, the horizon will be farther separated from the line which goes vertically upwards, and makes a greater angle therewith, it being about 38 degrees; this is the sun's meridian altitude at the time of equinox at London.

Again, rectify to the summer solstice by Problem xiii, and you will find the artificial horizon recede farther from the line which goes from London vertically upwards, and the angle it then makes, is about 62 degrees, which shews the sun's meridian altitude at the time of the summer solstice.

Hence, flows also the following arithmetical blem.

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PROBLEM XVIII. To find the meridian altitude

universally.

Add the sun's declination to the elevation of the

equator, if the latitude of the place, and the declination of the sun, are both on the same side.

If on contrary sides, subtract the declination from the elevation of the equator, and you obtain the sun's meridian altitude.

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Again, to the elevation of the equator at
London

Add the sun's greatest declination at the
time of the summer solstice

The sum is the sun's greatest meridian altititude at London

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PROBLEM XIX. Of the sun's azimuths, as compared with the artificial horizon.

The artificial horizon serves, also, to determine the sun's azimuths.

An azimuth of the sun is denominated from that point of the horizon, to which the sun, or a line going to the sun, is nearest.

Thus, if the sun, or a line going to the sun, be nearest the south-east point of the horizon, which

point is 45 degrees distant from the meridian, the sun's azimuth is an azimuth of 45 degrees, and the 'sun will appear in the south-east.

Imagine the sun, as we have done before, to be placed directly over the globe.

In which case, a line going to the sun from any place on the surface of the globe, will have a vertical direction, and will go from that place vertically upwards.

If, then, we apply the artificial horizon to any place, the point of this horizon, to which a vertical line is nearest, shews the sun's azimuth at that time.

It is observable, that the point of the horizon, to which such a vertical line is nearest, will be, at all times, that point which is most elevated.

To exemplify this, let the globe be in the position of a right sphere, and let the artificial horizon be applied to London.

When London is at the western edge of the broad paper circle, which situation represents the time when the sun appears to rise, the eastern point of the artificial horizon being then most elevated, shews that the sun at his rising is due east.

Turn the globe, till London comes to the eastern edge of the broad paper circle, then the western point of the artificial horizon will be most elevated, shewing that the sun sets due west.

Now, place the globe in the position of an oblique sphere; and, if London be brought to the eastern or western side of the broad paper circle, the ver-.

tical line will depart, more or less, from the east and west points; in which case, the sun is said to have more or less amplitude.

If the departure be northward, it is called northern amplitude; if southern, it is called southern amplitude.

In whatever position the globe be placed*, when London comes to the strong brass meridian, the most elevated part of the artificial horizon will be the south point of it.

Which shews that at noon the sun will always, and in all seasons, appear in the south.

OF THE ANCIENT DIVISIONS OF THE EARTH INTO ZONES AND CLIMATES.

Climates was a term used by the ancient astronomers to express a division of the earth, which, before the marking down the latitudes of countries into degrees and minutes was in use, served them for dividing the earth into certain portions in the same direction; so as to speak of any particular place with some degree of certainty, though not with due precision.

It was natural for the earliest observers to remark, for one of the first things, the diversity that there was in the sun's rising and setting; it was by this

*The globe is not supposed in this case, or under this view of things, ever to be elevated above the limits of the sun's deli neation.

they regulated what they called climates; which are a tract on the surface of the earth of various breadths, being regulated by the different lengths of time between the rising and setting of the sun in the longest day, in different places.

From the equator to the latitude of 661 north and south, a climate is constituted by the difference of half an hour in the length of the longest day; and this is sufficient for understanding the ancients. Between the polar circle and the pole, the length of the longest day, in one parallel, exceeds the length of the longest in the next by a month; but of these the ancients knew nothing.

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