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ESSAY II.

PART III.

THE USE OF THE

TERRESTRIAL GLOBE,

MOUNTED IJ>J THE BEST MANNER,

QF LONGITUDE AND LATITUDE, OF TERRESTRIAL ME • RIDIANS, AND THE PROBLEMS RELATING TO GlTUDE AKP LATITUDE,

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.ERIDIANS are circular lines, going over the earth's surface, from one pole to the other, and crossing the equator at right angles.

Whatever places these circular lines pass through, in going from pole to pole, they are the meridians of those places.

There are no places upon the surface of the earth, through which meridians may not be conceived to pass. Every place, therefore, is supposed to have a meridian line passing over its fcenith from north to jouth, and going through the poles of the world;

Thus, the meridian of Paris is one meridian; the meridian of London is another. This variety of meridians is satisfactorily represented on the globe, by the moveable meridian, which may be set to every individual point of the equator, and put directly eyer any particular place.

Whensoever we move towards the east or west, wr change our meridian; but we do not change our meridian if we move directly to the north or south.

The moveable meridian shews that the poles of the earth divide every meridian into two semicircles, one of which passes through the place whose meridian it is, the other through a point on the earth, opposite to that place.

Hence it is, that writers in geography and astronomy generally mean by the meridian of any place,, the semicircle which passes through that place; these, therefore, may be called the geographical meridians.

All places lying under the same semicircle, are said to have the same meridian; and the semicircle opposite to it, is called the opposite meridian, or sometimes the opposite pavt of the meridian.

From the foregoing definitions, it is clear that the meridian of any place is immoveably fixed to that place, and is carried round along with it by the rotation of the globe.

When the meridian of any place is by the revolution of the earth brought to point at the sun, it is noon, or mid-day, at that place.

The plane of the meridian of any place maybe imagined to be extended to the sphere of the fixed stars. When, by the motion of the earth, the plane of a meridian comes to any point in the heavens, as the sun, moon, &c. that point, &c. is then said to come to the meridian. It is in this sense that we generally use the expression of the sun or stars coming to, or passing over the meridian.

The time which elapses between the noon of any one day, in a given place, and the noon of the day following in the same place, is called a natural day.

All places which lie under the same meridian, have their noon, and every other hour of the natural day,, at the same time. Thus, when it rs one in the afternoon at London, it is also one in the afternoon at every place under the meridian of London.

In order to ascertain the situation of any point, there must first be a settled part of the earth's surface, from which to measure; and as the point to be ascertained may lie in any part of the earth's surface, and as this surface is spherical, the place from whence we measure must be a circle. It would be necessary, however, to establish two such circles; one to know how far any place may be east or west of another, the second to know its distance north or south of the given point, and thus determine its precise situation.

Hence, it has been customary for geographers to fix upon the meridian of some remarkable place, as ajirst meridian, or standard; and, to reckon the distance of any place to the east or west, or its longitude, by its first distance from the first meridian.

On English globes, this first meridian is made to pass through London. The position of this first meridian is arbitrary, because on a globe, properly speaking, there is neither beginning nor end. The first person, (whose works, at least, are come down to us) who computed the distance of places by longitudes and latitudes, was Ptolemy, about the year after Christ 140,

The longitude of any place is its distance from the first meridian, measured by degrees on the equator.

To find the longitude of a place, is to find what degree on the equator the meridian of that place crosses.'

All places that lie under the same meridian, are said to have the same longitude; all places that lie under different meridians, are said to have different longitudes ; this difference may be «ast or west, and, consequently, the difference of longitude between any two places, is the distance of their meridians from each other, measured on the equator.

Thus, if the meridian of any place cuts the equator in a point, which is fifteen degrees east from that point, where the meridian of London cuts the equator, that place is said to differ from London in longitude 15 degrees eastward.

Upon the terrestrial globe, there are 24 meridians, dividing the equator into 24 equal parts, which are the hour circles of the places through which they1 pass.

The distance of these meridians from each other is 15 degrees, or the 24th part of 36o degrees; thus, 15 degrees is equal to 6ne hour.

By the rotation of the earth, the plane of every meridian points at the sun, one hour after that meridian which is next to it eastward; and thus they successively point at the sun every hour, so that the planes of the 24 meridian semicircles being extended, 'pass through the sun in a natural day.

To illustrate this, suppose the plane of the strong brass meridian to coincide with the sun, bring London to this meridian, and then move the globe round, and you will find these 24 meridians successively pass under the strong brass meridian, at one hour's distance from each other; till in 24 hours the earth will return to the same situation, and the meridian of London will again coincide with the strong brass circle.

By passing the globe round, as in the foregoing' article, it will be evident to the pupil, that if one of these meridians, fifteen degrees east of London, comes to the strong brass meridian, or points at the sun one hour sooner than the meridian of London, a meridian that is 3O degrees east, comes two hour's sooner, and so on; and, consequently, they will have noon, and every other hour, «o much sooner than at London; while those, whose meridian is 15 degrees westward from London, will have noon, and every other hour of the day, one hour later than at London, and so on, in proportion to the difference of longitude. These definitions being well understood, the pupil will be prepared not only to solve, but see the rationale of the following problems.

Pkoblem 1. Tojind the longitude of any place on the globe.

The reader will find no difficulty in solving this problem, if he recollects the definition we have given of the word longitude, namely, that it is the distance of any place from the first meridian

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