9. The Visible Horizon.

The circle that bounds the view at sea.

10. The Sensible Horizon.

A plane that touches the Earth at the place of the observer continued to meet the heavens, and is perpendicular to a plumb line.

11. The Rational Horizon.

A circle whose plane passes through the centre of the Earth, and is parallel to the Sensible Horizon.

C is the centre of the earth; X Y a part of the heavens; S the true place of the Sun, and S, its apparent place caused by the refraction.

A B is the height of the eye; A D a tangent from the eye to the earth; then the circle through A perpendicular to a plumb line is the Sensible Horizon, E A H in the figure.

The circle through D, F D V, is the Visible Horizon.

And the circle through C, G C R, is the Rational Horizon.

12. Artificial Horizon and its use.

A tray of mercury, water, or any other liquid, generally covered with a glass shade to protect it from the wind. It is used for observing altitudes when the sea horizon cannot be used.

13. True Course of a ship.

The angle formed by the track of a ship and a true Meridian.

14. Magnetic Course.

The angle formed by the track of a ship and a Magnetic Meridian.

15. Compass Course.

The direction of the ship's head as shewn by the ship's standard compass, which is affected with Variation and Deviation.

Let T be the direction of the True North point, M that of the Magnetic, and C that of the Ship's Compass :—

Then S AT is the True Course; SA M is the Magnetic Course, and S A C the Ship's Compass Course.

M T is the Variation, C M is the Deviation, and T C is the Error of the Compass.

16. Variation of the Compass.

How much the N. of the needle is drawn to the right or left of the true North through the magnetism of the Earth.

17. Deviation of the Compass.

How much the N. of the needle is drawn to the right or left of the magnetic North through the iron in the ship.

18. The Error of the Compass.

The combined effect of the Variation and Deviation.

19. Leeway.

The angle between the line of the keel and the wake.

HS is the direction of the ship's head, W is the direction of the wind, L Y that of ship's wake:

Then, H L Y is the Leeway.

20. Meridian Altitude of a celestial object.

The arc of that particular vertical circle meridian between the object and the horizon. greatest altitude reached by the object that day.

Fig.5

Ꮓ

S

P

called the

It is the

H

E

N

R

The circle E ZQ N is a celestial Meridian, or circle of of Declination; in this case it is the observer's Meridian.

P is the Pole, Z the Zenith, HR the Horizon, E Q the Equator, and S the Sun.

Then, SH is the Meridian Altitude.

SE is the Declination.

PS is the Polar Distance.

and S Z is the Zenith Distance.

21. Azimuth.

The arc of the horizon between the meridian of the observer and the vertical circle passing through the centre of the object; or the bearing of an object from the N. or S.

The circle now represents the Horizon, with Z the Zenith in its centre. P is the N. pole.

S is the place of the Sun when rising, hence the angle EZ S, or the arc E S is the Amplitude.

S, is the position of the Sun at some time in the afternoon; then, D Z A, or the arc D A is the azimuth reckoning from the South, or N Z A, or arc N A, reckoning from the North. 22. Amplitude.

The arc of the horizon between the East point and the object at rising; or between the West point and the object when setting.

23. Declination.

An arc of a meridian or circle of declination between the object and the Equinoctial. ES in Fig. 5 is the Declination.