The student will recollect that the time shewn by a sun-dial is not the exact time of the day, as shown by a watch or clock (see Definitions 55, 56, and 57, page 12.) A good clock measures time equally, but a sun-dial (though used for regulating clocks and watches) measures time unequally. The following table will show to the nearest minute how much a clock should be faster or slower than a sun-dial; such a table should be put upon every horizontal sun-dial.

Dials may be constructed on all kinds of planes, whether horizontal or inclined: a vertical dial may be made to face the south, or any point of the compass; but the two dials already described are the most useful. To acquire a complete knowledge of dialling, the gnomonical projection of the sphere, and the principles of spherical trigonometry must be thoroughly understood; these preleminary branches may be learned from Emerson's Gnomonical Projection, and Keith's Trigonometry. The writers on dialling are very numerous; the last and best treatise on this subject is Emerson's.

CHAPTER II.

Problems performed by the Celestial Globe.

PROBLEM LXV.

To find the right ascension and declination of the sun,*

or a star.

Rule. Bring the sun or star to that part of the brass meridian which is numbered from the equinoctial towards the poles; the degree on the brass meridian is the declination, and the number of degrees on the equinoctial, between the brass meridian and the point Aries, is the right ascension.

Or, Place both the poles of the globe in the horizon, bring the sun or star to the eastern part of the horizon: then the number of degrees which the sun or star is northward or southward of the east, will be the declination north or south; and the degrees on the equinoctial, from Aries to the horizon, will be the right ascension.

Examples. 1. Required the right ascension and declination of a Dubhe, in the back of the Great Bear. Answer. Right Ascension 162° 49′, declination 62° 48′ N.

2. Required the right ascension and declinations of the following stars.

7, Algenib, in Pegasus.
a, Scheder, in Cassiopeia.
A, Mirach, in Andromeda.
a, Acherner, in Eridanus.
a, Monkar, in Cetus.
ß, Algol, in Perseus.
Aldebaran, in Taurus.
u, Capella, in Auriga.

8, Rigel, in Orion.

y, Bellatrix, in Orion. , Betelguese, in Orion. a, Canopus, in Argo Navis. a, Procyon, in the Little Dog. Y, Algorab, in the Crow. a, Arcturus, in Bootes. le, Vendemiatrix, in Virgo.

* The right ascension and declinations of the moon and the planets must be found from an ephemeris; because, by their continual change of situation, they cannot be placed on the celestial globe, as the stars are placed.

PROBLEM LXVI.

To find the latitude and longitude of a star.*

Rule. Place the upper end of the quadrant of altitude on the north or south pole of the ecliptic, according as the star is on the north or south side of the ecliptic, and move the other end till the star comes to the graduated edge of the quadrant; the number of degrees between the ecliptic and the star is the latitude; and the number of degrees on the ecliptic, reckoned eastward from the point Aries to the quadrant, is the longitude.

Or, elevate the north or south pole 6610 above the horizon, according as the given star is on the north or south side of the ecliptic; bring the pole of the ecliptic to that part of the brass meridian which is numbered from the equinoctial towards the pole; then the ecliptic will coincide with the horizon; screw the quadrant of altitude upon the brass meridian over the pole of the ecliptic keep the globe from revolving on its axis, and move the quadrant till its graduated edge comes over the given star: the degree on the quadrant cut by the star is its latitude; and the sign of the degree on the ecliptic cut by the quadrant shows its longitude.

Examples. 1. Required the latitude and longitude of a Aldebaran in Taurus.

Answer. Latitude 5o 28′ S. longitude 2 signs 6° 53′; or 6° 53′ in Gemini.

2. Required the latitudes and longitudes of the fol lowing stars.

a, Markab, in Pegasus.

, Scheat, in Pegasus.

, Vega, in Lyra.

Y, Rastaben, in Draco.

a, Fomalhaut, in the S. Fish. a, Antares, in the Scorpion.

a, Deneb, in Cygnus.

a, Altair, in the Eagle.

8, Albireo, in Cygnus.

a, Arcturus, in Bootes.
B, Pollux, in Gemini.
B, Rigel, in Orion.

* The latitudes and longitudes of the planets must be found from an

ephemeris.

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PROBLEM LXVII.

The right ascension and declination of a star, the moon, a planet, or of a comet, being given, to find its place on the globe.

Rule. Bring the given degrees of right ascension to that part of the brass meridian which is numbered from the equinoctial towards the poles; then, under the given declination on the brass meridian, you will find the star, or place of the planet.

Examples. 1. What star has 261° 29′ of right ascension, and 52° 27' north declination?

Answer. ẞ in Draco.

2. On the 20th of August, 1805, the moon's right as. cension was 91° 3′ and her declination 24° 48'; find her place on the globe at that time.

Answer. In the Milky Way, a little above the left foot of Castor. 3. What stars have the following right ascensions and declinations?

4. On the first of December, 1813, the moon's right ascension at midnight was 352° 21', and her declination 17° 25' S.; find her place on the globe.

5. On the first of May, 1805, the declination of Venus was 11°41' N. and her right ascension 31° 30′; find her place on the globe at that time.

6. On the 19th of January, 1805, the declination of Jupiter was 19° 29′ south, and his right ascension 238°. find his place on the globe at that time.

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