RIGHT ASCENSION AND DECLINATION.

97

Argo Navis, are on my meridian, at the same time every day, with a of Hydra, (Cor Hydra); and how is the plane passing through them marked on the equinoctial?

7. When the Southern "Pointers," a and y of Crux, are on the meridian of any place, what stars in Musca Australis, Corvus, Coma Bernices, and Draco are culminating; and what point of the equinoctial is culminating with them all?

8. When η of Boötes is on the meridian, what star in the Great Bear's Tail, what little star in Virgo, and what considerable star in Centaurus, are likewise very nearly on the meridian?

9. What stars in Ursa Minor, in Draco, in Corona Borealis, in Serpens, in Lupus, and in Triangulum Australe, are on the meridian with y of Libra; and what point of the equinoctial culminates with them?

10. What three stars in Hercules, what stars in Serpentarius, and what two remarkable stars in Ara, culminate every day about 5 minutes before S of Draco ?*

11. What stars in Draco, Cygnus, Sagittarius, and Pavo, come to the meridian with a of Aquila (Altair); and what degree of the equinoctial?

12. What degree of the equinoctial, and what small stars in Cepheus, Pegasus, Aquarius, and Grus, culminate every day with a of Piscis Australis, (or “ Fomalhaut") ?

PROBLEM XVI.

TERRESTRIAL GLOBE,

To find a star, or any heavenly body, having its Right Ascension and Declination given.

Conversely. Having a star, &c., given, to find its R. A. and Declination.

Repeat the following:

Sidereal Clock, (def. 53); Magnitude of a Star, (def. 5); Aberration, (def. 103); Nebula, (def. 106); Galaxy, (def. 107.)

Near the pole of the Ecliptic, and just preceding it; i, e. westward of it.

K

RULE.-Place the poles in the horizon, so that, in causing the globe to revolve westward, all the stars mapped upon it may pass under review. Place the thumb nail (of the right or left hand according to position) on the given declination, and bring the given right ascension to the brass meridian ; then, under the given declination will be the star required.

Conversely. If a given star be brought to the brass meridian, the declination will be that found exactly over the star; and the R. A. will be cut by the brass meridian on the equinoctial.

1. Find the stars answering to the following R. A. and Declination?

2. Find the R. A. and Declination of the following stars?

1st. The R. A. in hours or

2nd. The R. A. in degrees.

a of Crux.

B of Corvus.

of Ursa Major, “Mizar.” a of Ursa Minor, Pole Star. y of Grus.

3. The telescope of my Transit instrument is elevated to 23° 26' N. declination, and my sidereal clock is showing 3 hours 31 minutes: what star shall I see at the intersection of the wires which point out the centre of the tube, if I look through it just six minutes and three quarters after this?

DIFFERENCE OF LONGITUDE.

99

69

PROBLEM XVII.

TERRESTRIAL GLOBE.

To find the difference of Longitude of any two places; and the hour of the day at the one place being given, to find the hour at the other place.

Learn S. T. V. W. on page 12, and X. on page 13.

RULE. Find the longitudes of both places, beginning with that place which is eastern of the two. If the longitudes be both of the eastern hemisphere, or both of the western, subtract the less longitude from the greater; but, if the one longitude be of the eastern, and the other of the western hemisphere, their sum will be the difference of longitude, unless it exceed 180 degrees.

If the sum of the two longitudes exceed 180°, take that sum from 360°, and the remainder will show the difference of longitude.

N. B. The difference of longitude may be turned into minutes of time by multiplying the degrees by 4; or into hours by dividing them by 15.

What is the difference of longitude between the following places: Pekin and Lisbon; Botany Bay and Cairo; Port Royal and Owhyhee; Naples and Lassa, (Thibet); Geneva and Lima; Philadelphia and Venice; Paris and Rome; Lisbon and Canton; Astracan and Barbadoes; Mexico and Otaheite; the North Foreland and the Isle of Wight; Dublin and Edinburgh; what is the difference of time between these places, and which one, of every two of them, has the time in advance?

ANGULAR MOTION, (PROPERLY, ANGULAR VELOCITY,)

Of any body moving about a centre, (as distinguished from its real velocity,) is measured by the number of degrees through which it passes in a given time, as an hour, a second, &c.

S

Thus the knots E. M. P. we have supposed to be made in the skipping-rope, have, all of them, the same angular velocity, although varying in their real velocity as their respective distances from their centres of motion, viz. the chest, arm, and wrist.

If my watch, which performs correctly, after having had its minute hand, three quarters of an inch long, set by St. Paul's clock, could be hung up in the exact centre of the face of that clock, it is plain that the direction of the pointers, of the watch and of the clock, could agree constantly only because of their corresponding angular ve locity. But the pointer, or extremity, of the clock-hand which measures 8 feet, or 96 inches, must have a real velocity according with that length; and move over rather more space in one half minute, than the pointer of my watch-hand does in an hour's complete revolution.

Apply this, in the Problem next succeeding, to the velocities of different latitudes; and in a Problem of the third Section, we shall consider how the Divine Contriver has adapted this means, amongst others, to the production of fertility and convenience on our planetary habitation.

ROTATION OF DIFFERENT LATITUDES.

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PROBLEM XVIII.

TERRESTRIAL GLOBE.

To find at what rate per minute, per hour, &c., the inhabitants of any given Latitude are carried eastward by the rotation of the earth; and the length of a degree of Longitude in that Latitude.

Repeat the Tables on page 91. Also K. L. and P. on pp. 1 and 2, and Hour Circles," (def. 26.)

Remark. If our earth performed its diurnal revolution in 360 hours, that is, if it revolved at the rate of one degree per hour, or in 60 minutes of time, it is evident that the inhabitants of the large circle called the Equator, each degree of which measures 60 Geographical miles, would be carried, by this revolution, through one of these Geographical miles per minute; but the earth revolves, in about 24 hours, or 15 times as fast as this, i. e. at the rate of 15 degrees per hour. The inhabitants of the Equator are, therefore, carried at the rate of 15 Geographical miles per minute. Now the degrees of the quadrant of altitude are degrees of the Equator: hence, we compare these 15 degrees of the Equator, or its 15 miles per minute, with the 15 degrees of any given latitude, and, therefore, with the rate of motion in that latitude, in the following

manner :

RULE. Place the graduated edge of the quadrant in the given latitude, and parallel to the equator, between any two meridians that differ in longitude 15°; and the number of degrees of the quadrant intercepted between them, will show how the rate of motion decreases, or the Geographical miles per minute through which the inhabitants of the given latitude are carried. If the rate required be per hour, multiply by 60. When it is required to find the Geo