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 east 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 they

pass.

The distance of these meridians from each other is 15 degrees, or the 24th part of 360 degrees; thus, 15 degrees is equal to one 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 30 degrees east, comes two hour's sooner, and so on; and, consequently, they will have noon, and every other hour, so 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.

PROBLEM 1. To find 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 mea

sured on the equator. Therefore, either set the moveable meridian to the place, or bring the place under the strong brass meridian, and that degree of the equator, which is cut by either of the brazen meridians, is the longitude in degrees and minutes, or the hour and minute of its longitude, expressed in time.

As the given place may lie either east or west of the first meridian, the longitude may be expressed accordingly.

It appears most natural to reckon the longitude always westward from the first meridian; but it is customary to reckon one-half round the globe eastward, the other half westward from the first meridian. To accommodate those who may prefer either of these plans, there are two sets of numbers on our globes the numbers nearest the equator increase westward, from the meridian of London, quite round the globe to 360', over which another set of numbers is engraved, which increase the contrary way; so that the longitude may be reckoned upon the equator, either east or west.

Example. Bring Boston, in New England, to the graduated edge of either the strong brass, or of the moveable meridian, and you will find its longitude in degrees to be 704, or 4 hours 42 minutes in time; Rome, 12 degrees east, or 50 minutes in time; Charles-Town, North-America, is 79 degrees 50 minutes west.

PROBLEM II.

To find the difference of longitude between any two places.

If the pupil understands what is meant by the difference of longitude, the rule for the solution of this problem will naturally occur to his mind. Now the difference of longitude between any two places is the quantity of an angle (at the pole) made by the meridians of those places measured on the equator. To express this angle upon the globe, bring the moveable meridian to one of the places, and the other place under the strong brass circle, and the required angle, is contained between these two meridians, the measure or quantity of which is to be counted on the equator.

Example. I find the longitude of Rome to be 12 east, that of Constantinople to be 29; the difference is 17 degrees. Again, I find Jerusalem has 35 degrees 25 minutes east longitude from London; and Pekin in China, 116 degrees 52 minutes east longitude; the difference is 81 degrees 27 minutes; that is, Pekin is 81 degrees 27 minutes east longitude from Jerusalem; or Jerusalem is 81 degrees 27 minutes west longitude from Pekin.

If one place is east, and the other west of the first meridian, either find the longitude of both places westward, by that set of numbers which increase westward from the meridian of London to 360 degrees; and the difference between the number thus found, is the answer to the question: or, add

the cast and west longitudes, and the sum is the difference of longitude; thus the longitude of Rome is 12 degrees 30 minutes east of Charles-Town, 79 degrees 50 minutes west; their sum, 91 degrees 20 minutes, is the difference required.

It may be proper to observe here, that the differ ence in time is the same with the difference of longitude; consequently, that some of the following problems are only particular cases of this problem, or readier modes of computing this difference.

PROBLEM III. To find all those places where it is noon, at any given hour of the day, at any given place.

General rule. Bring the given place to the brass meridian, and set the index to the uppermost XII; then turn the globe till the index points to the given hour, and it will be noon to all the places under the meridian.

As the diurnal motion of the earth is from west to east, it is plain that all places which are to the east of any meridian, must necessarily pass by the sun before a meridian, which is to the west, can arrive at it.

N. B. As in my father's globes, the XII, or first meridian, passes through London, you have only to bring the given hour to the east of London, if in the morning, to the brass meridian, and all those places which are under it, will have noon at the given hour;