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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 it& longitude in degrees to be fO^, or 4 hours 42 minutes in time; Kome, 12.1 degrees east, or 5O minutes in time; Charles-Town, North-America, is 79 degrees 5O minutes west.
Problem n. To Jind 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 ] 2? 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, I16 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 3fio degrees; and the difference between the number thus found, is the answer to the question: or, add
the east and west longitudes, and the sum is the'difference of longitude; thus the longitude of Rome is 12 degrees 3O minutes east of Charles-TowiJ, 79 degrees 5O minutes west; their sum, 91 degrees 2O minutes, is the difference required.
It may be proper to observe here, that the difference 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 in. To Jlnd 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 honr, 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 wrest, can arrive at it.
N. B. As in my father's globes, the XII, or first meridian, pisses 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; but, bring the given hour westward of London, if it be in the afternoon.
When it is 4 hours 50 minutes in the afternoon at Paris, it is noon at New Britain, New England, Sti Domingo, Terra Firma, Peru, Chili, and Terra del Fuego.
When it is 7 hours 50 minutes in the morning at Ispahan, it is noon at the middle of Siberia, Chinese Tartary, China, Borneo.
Problem iv. fPhen it is noon at any place, to find what hour of the day it is at any other place.
Rule. Bring the place at which it is noon to the strong brass meridian, and set the hour index to the uppermost XII, and then turn the globe about till the other place comes under the strong brass meridian, and the hour index will shew upon the equator the required hour. If to the eastward of the place where it is noon, the hour found will be in the afternoon; if to the westward, it will be in the forenoon.
Thus, when it is noon at London, it is 50 minutes past XII at Rome; 32 minutes past VII in the evening at Canton, in China; 15 minutes past VII in the morning at Quebec, in Canada.
Problem V. The hour being given at any place, to tell what hour it is in any other part of the world.
Rule. Bring the place where the time is required
under the strong brass meridian, set the hour index , to the given time, then turn the globe, till the other place is under the brass meridian, and the horary index will point to the hour required.
Thus, suppose we are at London at IX o'clock in the morning, what is the time at Canton, in China? Answer, 31 minutes past IV in the afternoon. When it is IX in the evening at London, it is about 15 minutes past IV in the afternoon at Quebec, in Canada.
Thus, also, when it is III in the afternoon at London, it is 18 minutes past X in the forenoon at Boston. When it is VI in the morning at the Cape of Good Hope, it is 7 minutes after midnight at Quebec.
I have always observed, that the equator divide* the globe into two hemispheres, the northern and the southern.
The latitude of a place is its distance from the equator towards the north or south pole, measured by degrees upon the meridian of the place.
All places, therefore, that lie under the equator, are said to have no latitude.
All other places upon the earth, are said to be in north or south latitude, as they are situated on the north or south side of the equator; and the latitude of any place will be greater or less, according as it is further from, or nearer to the equator.