PROBLEM XI. To illustrate, by the globe, so much of the equation of time as is in consequence of the sun's apparent motion in the ecliptic.

Bring every tenth degree of the ecliptic to the graduated side of the strong brass meridian, and you will find that each tenth degree on the equator will not come thither with it; but in the following order from Y to, every tenth degree of the ecliptic comes sooner to the strong brass meridian than their corresponding tenths on the equator; those in the second quadrant of the ecliptic, from to, come later; from to V, sooper; and from V to Y, later; whilst those at the beginning of each quadrant come to the meridian at the same time; therefore the sun and clock would be equal at these four times, if the sun was not longer in passing through one half of the ecliptic than the other, and the two inequalities joined together, compose that difference which is called the equation of time.

These causes are independent of each other, sometimes they agree, and at other times are contrary to one another.

The inequality of the natural day is the cause that clocks or watches are sometimes before, and sometimes behind the sun.

A good and well-regulated clock goes uniformly on throughout the year, so as to mark the equal hours of a natural day of a mean length; a sun-dial marks the hours of every day in such a manner, that

every

hour is a 24th part of the time between the noon of that day, and the noon of the day immediately following. The time measured by a clock is called equal or true time; that measured by the sundial apparent time.

THE USE OF THE CELESTIAL GLOBE, IN PROBLEMS RELATIVE TO THE FIXED STARS.

The use of the celestial globe is in no instance more conspicuous than in the problems concerning the fixed stars. Among many other advantages, it will, if joined with observations on the stars themselves, render the practice and theory of other problems easy and clear to the pupil, and vastly facilitate his progress in astronomical knowledge.

The heavens are as much studded over with stars in the day as in the night; only they are then rendered invisible to us by the brightness of the solar rays. But when this glorious luminary descends below the horizon, they begin gradually to appear; when the sun is about 12 degrees below the horizon, stars of the first magnitude become visible; when he is thirteen degrees, those of the second are seen; when fourteen degrees those of the third magnitude appear; when fifteen degrees, those of the fourth present themselves to view; when he is descended about eighteen degrees, the stars of the fifth and sixth magnitude, and those that are still smaller, be come conspicuous, and the azure arch sparkles with all its glory.

PROBLEM XII. To find the right ascension and declination of any given star.

Bring the given star to the meridian, and the degree under which it lies is its declination; and the point in which the meridian intersects the equinoctial is its right ascension. Thus the right ascension of Sirius is 99°, its declination 16° 25′ south; the right ascension of Arcturus is 211° 32', its declination 20° 20′ north.

The declination is used to find the latitude of places; the right ascension is used to find the time at which a star or planet comes to the meridian; to find at any given time how long it will be before any celestial body comes to the meridian; to determine in what order those bodies pass the meridian; and to make a catalogue of the fixed stars.

PROBLEM XIII.

To find the latitude and longitude of a given star.

Bring the pole of the ecliptic to the meridian, over which fix the quadrant of altitude, and, holding the globe very steady, move the quadrant to lie over the given star, and the degree of the quadrant cut by the star is its latitude; the degree of the ecliptic cut at the same time by the quadrant is the longitude of the star.

Thus the latitude of Arcturus is 30° 30', N. its Longitude 20' 20' of Libra: the latitude of Ca

pella is 22° 22′ north, its longitude 18° 8' of Gemini.

The latitude and longitude of stars is used to fix precisely their places on the globe, to refer planets and comets to the stars; and, lastly, to determine whether they have any motion, whether any stars vanish, or new ones appear,

PROBLEM XIV. The right ascension and decli nation of a star being given, to find its place on the globe.

Turn the globe till the meridian cuts the equinoctial in the degree of right ascension. Thus, for example, suppose the right ascension of Aldebaran to be 65° 30', and its declination to be 16° north, then turn the globe about till the meridian cuts the equinoctial in 65° 30′, and under the 16° of the meridian, on the northern part, you will observe the star Aldebaran, or the Bull's eye.

PROBLEM XV. To find at what hour

To find at what hour any known star

passes the meridian at any given day.

Find the sun's place for that day in the ecliptic, and bring it to the strong brass meridian, set the horary index to XII o'clock, then turn the globe till the star comes to the meridian, and the index will mark the time. Thus, on the 15th of August, Lyra comes to the meridian at 45 minutes past VIII in the evening. On the 14th of September, the brightest

Y

of the Pleiades will be on the meridian at IV in the

morning.

This problem is useful for directing when to look for any star on the meridian, in order to find the latitude of a place, to adjust a clock, &c.

PROBLEM XVI. To find on what day a given star will come to the meridian at any given hour.

Bring the given star to the meridian, and set the index to the proposed hour; then turn the globe till the index points to XII at noon. and observe the degree of the ecliptic then at' the meridian; this is the sun's place, the day answering to which may be found on the calendar of the broad paper circle.

By knowing whether the hour be in the morning or afternoon, it will be easy to perceive which way to turn the globe, that the proper XII may be pointed to; the globe must be turned towards the west, if the given hour be in the morning; towards the east, if it be afternoon.

Thus, Arcturus will be on the meridian at III in the morning on March the 5th, and Cor Leonis at VIII in the evening on April the 21st.

PROBLEM XVII. To represent the face of the heavens on the globe for a given hour on any day of the year, and learn to distinguish the visible fixed

stars.

Rectify the globe to the given latitude of the place