of right ascension on the equator, though the sun or stars may be far from the equator.

By these you may also compute on the earthly globe what hour it is at any place in the world, by having the true hour given at any other place, and by changing the degrees of their difference of longitude into hours.

But since several questions or problems that relate to the bour, cannot be so commodiously resolved by these few meridians or hour-lines, because every place on the earth hath its proper meridian where the sun is at 12 o'clock, therefore there is a brass dial-plate fixed at the north-pole in the globe, whose 24 hours do exactly answer the 24 hour circles which might be drawn on the globe: now the dial being fixed, and the pointer being moveable, this answers all the purposes of having an infinite number of hour circles drawn on the globe, and fitted to every spot on the heavens or the earth. For the pointer or index may be set to 12 o'clock, when the sun's true place in the heavens, or when any place on the earth is brought to the brass meridian, and thus the globe moving round with the index naturally represents, and shews by the dial-plate the 24 hours of any day in the year, or in any particular town or city.

Note, The upper 12 a clock is the hour of noon, the lower 12 is the midnight hour, when the globe is fixed for any particular latitude where there are days and nights.

The declination of the sun or stars is their distance from the equator toward the north or south pole, measured on the meridian; and it is the same thing with latitude on the earthly globe.

Note, the sun in the vernal or autumnal equinoxes, and the stars that are just on the equator have no declination.

Parallels of declination are lines parallel to the equator, the same as the parallels of latitude on the earthly globe. In the heavens they may be supposed to be drawn through each degree of the meridian, and thus shew the declination of all the stars; or they may be drawn through every degree of the ecliptic, and thus represent the sun's path, every day in the year. These parallel lines also would lead the eye to the degree of the sun's or any particular star's declination marked on the meridian.

The declination is called north or south declination according as the sun or stars lies northward or southward from the equator.

Observe here, that as any place, town, or city on earth is found determined by the parallel of its latitude; crossing its line of longitude; so the proper place of the sun or star in the heavens is found and determined by the point where its parallel of declination crosses its meridian or line of right ascension;

which indeed are but the self same things on both the globes, though astronomers have happened to give them different names.

Note, The sun's utmost declination northward in our summer is but 23 degrees; and it is just so much southward in our winter; for then he returns again; there the tropics are placed which describe the path of the sun when farthest from the equator, at midsummer, or midwinter; these two tropics are his parallels of declination on the longest and shortest day.

While the sun gains 90 degrees on the ecliptic, (which is an oblique circle) in a quarter of a year, it gains but 23 degrees of direct distance from the equator measured on the meridian; this appears evident on the globe, and may be represented thus in figure v.

Let the semicircle Y P be the meridian of the northern hemisphere, the line YC be the equator or the sun's path at Aries and Libra, the arch Y the ecliptic, the line T 0 the summer tropic, the line a e the sun's path when it enters Gemini and Leo, the line n s the sun's path when it enters Taurus and Virgo; then it will appear that in moving from Y to the sun gains 30 degrees in the ecliptic, in about a month, and at the same time 12 degrees of declination, viz. from Y to n. Then moving from 8 to II in a month more it gains 30 degrees on the ecliptic, and 8 degrees of declination, viz. from n to a. Then again from II to in a month more it gains 30 degrees on the ecliptic, and but 34 degrees of declination, viz. from a to T. I might also shew the same difference between its declination and its motion on the ecliptic in its descent from to . m, and .

By drawing another scheme of the same kind below the line YC, we might represent the sun's descent towards the winter solstice, and its return again to the spring; and thereby shew the same differences between the sun's declination and its motion on the ecliptic in the winter half year as the present scheme shews in the summer half-year.

Hereby it is evident how it comes to pass, that the sun's deelination alters near half a degree every day just about the equinoxes; but it scarce alters so much in 10 or 12 days on each side of the solstices; aud this shews the reason why the length of days and nights changes so fast in March and September, and so exceeding slowly in June and December; for according to the increase of the sun's declination in summer, its semidiurnal arc* will be larger, and consequently it must be so much longer before it comes to its full height at noon, and it stays so much longer above the horizon before it sets.

The "diurnal arc" is that part of the circle or parallel of declination which is above the "horizon;" and the half of that part is called the “ semidiurnal are."

Thus while the sun's declination increases or decreases by slow degrees, the length of the days must increase and decrease but very slowly; and when the sun's declination increases and decreases swiftly, so also must the length of the days; all which are very naturally and easily represented by the globe.

SECT. VII.-Of Longitude and Latitude on the Heavenly Globe, and of the Nodes and Eclipses of the Planets.

THE longitude and latitude in astronomy are quite dif ferent things from longitude and latitude in geography, which is ready to create some confusion to learners.

The longitude of the sun or any star is its distance from the point aries eastward, measured in the ecliptic. This is a short way of describing it, and agrees perfectly to the sun; but in truth a star's longitude is its distance eastward from a great arch drawn perpendicular to the ecliptic through the point aries, and measured on the ecliptic.

We do not so usually talk of the sun's longitude, because we call it his place in the ecliptic, reckoning it no farther backward than from the beginning of the sign in which he is. So the 25th day of June, we say the sun is in the 14th degree of cancer, and not in the 104th degree of longitude.

The latitude of a star or planet is its distance from the ecliptic, measured by an arch, drawn through that star perpendicular to the ecliptic.

Longitude and latitude on the heavenly globe bear exactly the same relation to the ecliptic as they do on the earthly globe to the equator. As the equator is the line from which the latitude is counted, and on which the longitude is counted on the earthly globe, so the ecliptic is the line from which the latitude, and on which the longitude are counted on the heavenly globe.

And thus the lines of latitude in the heavenly globe are all supposed parallels to the ecliptic, and the lines of longitude cut the ecliptic at right angles, and all meet in the poles of the ecliptic, bearing the same relation to it as on the earthly globe they do to the equator.

The latitude of a star or planet is called northern or southern as it lies on the north or south side of the ecliptic.

The sun has no latitude, because it is always in the ecliptic. This relation of latitude therefore chiefly concerns the planets and stars.

The fixed stars as well as the planets have their various longitudes and latitudes; and their particular place in the hea vens may be assigned and determined thereby, as well as by their right ascension and declination which I mentioned before; and astronomers use this method to fix exactly the place of a

star. But I think it is easier for a learner to find a star's place by its declination, and right ascension; and the common astronomical problems seem to be solved more naturally and easily by this method.

It may be here mentioned, though it is before its proper place, that the several planets, viz. Saturn, Jupiter, Mars, Venus, Mercury, and the Moon, make their revolutions at very different distances from the earth, from the sun, and from one another; each having its distinct orbit or path nearer or farther And as each of their orbits is at vastly different distances, so neither are they perfectly parallel to one another, nor to the ecliptic or yearly path of the sun.

Thence it follows that these planets have some more, some less latitude, because their orbits or paths differ some few degrees from the sun's path, and intersect or cross the ecliptic, at two opposite points in certain small angles of two, three, four or five degrees, which points are called the nodes.

The node where any planet crosses the ecliptic ascending to the northward is called the dragon's head, and marked thus. Where the planet crosses the ecliptic descending to the southward, it is called the dragon's tail and marked thus .

It is very difficult to represent the latitude of the planets in their different orbits either upon a globe, or upon a flat or plain surface; the best way that I know is, to take two small hoops of different sizes, as in figure XI. and thrust a straight wire co through them both in the two opposite parts of their circumference: Then turn the innermost hoop (which may represent the path of the moon) so far aside or obliquely as to make an angle of 5 degrees with the outermost hoop, (which represents the sun's path.) Thus the two points c and o or N and where the wire joins the hoops, are the two nodes or the points of intersection.

This difference of orbits of the planets and their intersections, or nodes, may be represented also by two circular pieces. of pasteboard as in figure XII. When the less (whose edge represents the moon's orbit,) is put half way through a slit A, B, that is made in the diameter of the larger (or the sun's orbit, and then brought up near to a parallel or level with the larger within 5 degrees. Thus the two nodes will be represented by A and B.

* Astronomers know that not only the 12 constellations of the zodiac, but also all the fixed stars seem to move from the west toward the east about 50 in a year, or one degree in 72 years, in circles parallel to the ecliptic. Therefore their declination is a little altered in 72 years time, that being measured from the equator: But their latitude never alters, that being measured from the ecliptic: And upon this account astronomers use the latitude rather than the declination in their measures, because it abides the same for ever.

If the moon's path and the sun's were precisely the same, or parallel circles in the same plane, then at every new moon the sun would be eclipsed by the moon's coming between the earth and the sun And at every full moon the moon would be eclipsed by the earth's coming been the sun and the moon. But since the planes of their orbits or paths are different, and make angles with each other there cannot be eclipses but in or near the place where the planes of their orbits or paths intersect or cross each other.

In or very near these nodes, therefore, is the only place where the earth or moon can hide the sun or any part of it from each other, and cause an eclipse either total or partial; And for these reasons the orbit or path of the sun is called the ecliptic.

The eclipses of other planets, or of any part of the sun by their interposition, are so very inconsiderable as deserve not our present notice.

SECT. VIII. Of Altitude, Azimuth, Amplitude, and various Risings and Settings of the Sun and Stars.

THE altitude of the sun or star is its heighth above the horizon, measured by the degrees on the quadrant of altitudes.

As the height of the sun at noon is called its meridian altitude, or its culminating, so the height of the sun in the east or west is sometimes called its vertical altitude.

The quadrant of altitudes is a thin label of brass, with a nut and skrew at the end of it, whereby it is fastened to the meridian at the zenith of any place; now by bending this down to the horizon, you find the altitude of any star or point in the heavens, because the label is divided into 90 degrees counting from the horizon upward.

Circles parallel to the horizon, supposed to be drawn round the globe, through every degree of the quadrant of altitudes less and less till they come to a point in the zenith, are called parallels of altitude, or sometimes in the old arabic name almicantars. But these can never be actually drawn on the globe, because the horizon and zenith are infinitely variable, according to the different latitudes of places. In the vith figure, suppose z to be the zenith, N the nadir, HR the horizon, and the strait lines d b, fg, km, will represent the parallels of altitude.

Note, The sun being always highest on the meridian, or at noon, it descends in an arch towards the horizon in order to set, by the same degrees by which it ascended from the horizon after its rising. Stars and planets rise and set and come to the meridian at all different hours of the day or night according to the various seasons of the year, or according to the signs in which the planets are,