7. The amplitude of any object in the heavens, is an arc of the horizon, contained between the centre of the object when rising, or setting, and the east or west points of the horizon. Or, it is the distance which the sun or a star, rises from the east, and sets from the west, and is used to find the variation of the compass at sea.

In our summer, the sun rises to the north of the east, and sets to the north of the west; and in the winter, it rises to the south of the east, and sets to the south of the west, except at the time of the equinoxes.

8. The quadrant of altitude is a thin slip of brass divided upwards from 0 to 90 degrees; and downwards from 0 to 18 degrees, and, when used, is generally screwed to the brass meridian.

The upper divisions are used to determine the distances of places on the earth, the distances of the celestial bodies, their altitudes, &c.; and the lower divisions are applied to finding the beginning, end, and duration of twilight.

9. The hour circles are two small circles on the globe, placed at the north and south poles, having the hours of the day delineated upon them, with an index to each, pointing to any particular time.

10. The indexes are two moveable pointers fixed at the north and south poles, which are the centres of the hour circles.

11. Every hour answers to 15 degrees of the equator, and distance is by that means reduced into time at pleasure.

Hence every degree answers to four minutes of time, every half degree to two minutes, and every quarter degree to one minute.

2.

12. The compass, usually called the Mariner's Compass, is a representation of the horizon, and consists of a circular brass box, which contains a paper card, divided into 32 equal parts, and fixed

on a magnetical needle that generally turns towards the north. Each point of the compass contains 11o 15', or 11 degrees, being the 32d part of 360 degrees.

The compass is used for setting the terrestrial globe north and south; but care must be taken to make a proper allowance for the variation. The compass is also used by seamen to direct and ascertain the course of their ships.

13. It is proper to observe, that the needle does not always point directly north, and that it is also subject to a small variation: its deviation from the north point of the horizon is called its declination; and the change of its declination, is properly called the variation of the needle. This term, however, is usually used to signify the declination itself.

At present, at London, the north end of the needle points about 24 degrees towards the west of the true north point of the horizon, but at the North Cape it points only about 19 towards the west; while in some parts of Davis's Straits its direction is more than 61⁄2 points towards the west, and at Cape Horn it points about 22 degrees towards the east of the truth north. When the north point of the compass is to the east of the true north point of the horizon, the declination is called east; if it be to the west, the declination is west.

What is the distinction between the rational and sensible horizon ?

What is the wooden horizon, and how is it divided?

What is the amplitude of a celestial object, and what is its principal use ?

What is the quadrant of altitude, how is it divided, and what is its use?

What are the hour circles, and where are they situated on the globe ?

What is the compass, how is it divided, and what is its use on the globe ?

hes the needle always point directly north ?

What is the deviation of the needle from the north point of the horizon called?

What are the indexes, and where are they fixed? How many degrees of the equator answers to an hour, and how many minutes of time corresponds to a degree ?

CHAPTER V.

Definitions and terms belonging to the terrestrial globe. 1. The east, west, north, and south points of the horizon, are called cardinal points.

When the days and nights are equal, that is, when the sun is in the equinoctial; the point of the horizon where the sun rises is called the east; and the point where he sets is called the west; the point of the horizon towards which the sun appears at noon to those situated in the northern hemisphere, is called the south; and the point of the horizon directly opposite to the south is called the north.

2. The equinoctial and solstitial points are called the cardinal points of the ecliptic; and the cardinal signs are Aries, Cancer, Libra, and Capricorn.

The time when the sun is at the equinoctial point, in his passage from the south to the north side of the equator, is called the vernal equinox; and the time when he is at the other equinoctial point, is called the autumnal equinox.

The time when the sun is at the northern solstitial point, is called the summer solstice; and the time when he is at the southern solstitial point, is called the winter solstice.

3. The cardinal points in the heavens are the zenith, the nadir, and the points where the sun rises and sets.

4. It is found by experiment that the plumb line, when the plummet is freely suspended, and is at rest, is perpendicular to the free surface of still

water.

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Hence, a straight line in the direction of gravity at any place on the earth's surface, is also perpendicular both to the sensible and rational horizons of that place.

5. If at any place on the earth's surface a straight line, in the direction of gravity, be produced both ways to the heavens, the point in which it cuts the celestial sphere, exactly over our heads, is called the zenith of the place; and the point in which it cuts the opposite part of the sphere, or directly under our feet, is called the nadir.

Hence, the zenith is the elevated pole of our rational horizon, and the nadir, being diametrically opposite to the zenith, is the depressed pole.

6. Azimuth, or vertical circles, are imaginary great circles passing through the zenith and nadir, and cutting the horizon at right angles.

The altitudes of the heavenly bodies are measured on these circles: they may be represented by screwing the quadrant of altitude on the zenith of any place, and making the other end move along the horizon of the globe.

7. That azimuth circle which passes through the east and west points of the horizon, is called the prime vertical.

This circle is always at right angles with the brass meridian, which may be considered as another vertical circle passing through the north and south points of the horizon.

8. The azimuth of any object in the heavens is an arc of the horizon, contained between a vertical circle passing through the object, and the north or south points of the horizon.

The azimuth of the sun, at any particular hour, is used at sea for finding the variation of the compass.

9. The altitude of any object in the heavens is an arc of a vertical circle, contained between the centre of the object and the horizon. When the object is on the meridian, this arc is called the meridian altitude.

10. The zenith distance of any celestial object is an arc of a vertical circle, contained between the centre of that object and the zenith; or, it is what the altitude of the object wants of 90 degrees. When the sun is on the meridian, this arc is called the meridian zenith distance.

11. The polar distance of any celestial object is an arc of a meridian, contained between the centre of that object and the pole of the equinoctial.

QUESTIONS.

What are the cardinal points of the horizon? What are the cardinal points of the ecliptic, and

what are the cardinal signs?

What are the cardinal points in the heavens ?

What is the zenith, and of what circle is it the

pole?

What is the nadir, and of what circle is it the pole?

What are the azimuth or vertical circles, and what is their use ?

What is the prime vertical?

What is the azimuth of a celestial object?

What is the altitude of any object in the heavens, and what is the meridian altitude of the sun, a star, or planet ?

What is the zenith distance of a celestial object? What is the polar distance of a celestial object?