The following questions are set down for the learner's ex

ercise:

Quest. I. Being at sea, in latitude 40° 38′ N. in the afternoon, the sun's altitude was observed to be 20° 46, when his declination was 17° 10' S. what was the sun's azimuth at that time?

Ans. 137° 50' from the north.

Quest. II. What is the sun's true azimuth in lat. 26° 30' N. in the forenoon, when his altitude is 24° 28', and his declination 22° 40′ N.?

Ans. 75° 48′ from the north point of the compass.

Quest. III. At the island of St. Helena, the sun's altitude was observed to be 30° 22' in the forenoon, his declination being then 22° 58' S. required the azimuth at that time.

Ans. 72° 24 from the south, or 107° 36' from the north.

Quest. IV. What is the bearing of the star Aldebaran at the Cape of Good Hope, when its altitude is 22° 25' ?

Ans. 130° 20' from the south, or 49° 40' from the north.

Having found the sun's true amplitude or azimuth by the preceding methods, &c. magnetic amplitude or azimuth by observation, it is evident, that when they agree there is no variation; but when they disagree, then if the true and observed amplitudes be both of the same name, that is, both north, or both south, their difference is the variation; but if the true and observed amplitudes be of different names, that is, one north and the other south, their sum is the variation. Again, if the true and observed azimuths be both on the east, or both on the west side of the meridian, their difference is the variation; but if the true and observed azimuths be one on the east and one on the west side of the meridian, their un gives the variation; and to know whether the variation i easterly or westerly, observe this general

RULE.

Let the observer's face be turned to the sun; then if the true amplitude or azimuth be to the right hand of the magnetic, or observed, the variation is easterly; but if to the left hand, westerly. EXAMPLE I.

Suppose the sun's magnetic amplitude at rising is found to be E. 26 12 N. but the true is found to be E. 14° 20' N.; required the variation.

which is easterly, because in this case the true amplitude is to the right of the observed.

With the cord of 60 describe a circle to represent the compass, through which draw the north, south, cast, and west lines: take the amplitude at rising, 26° 12' from the line of cords, and setting it from E. towards N. and likewise the true amplitude 14° 20, and set it from E. towards N. as before: the difference of these two angles, or between the true and magnetic amplitude, viz. 11° 52′ is the variation. Now suppose yourself placed at the centre of the horizon represented by the compass, and looking towards the magnetic amplitude at the sun's rising, it is plain that the true amplitude found by calculation is towards the right hand of the observed, which shows the variation is 11° 52' E. and must be allowed to the right hand in all courses steered, before they can be put in the Traverse Table, or bearings taken by the compass."

EXAMPLE II.

Suppose the sun's true amplitude at setting be W. 34° 26' S. and magnetic amplitude W. 23' 13' S. required the variation. Since they are both of the same name,

which is westerly, because the true amplitude is to the left of the observed in this case.

2,

* Let N. E. S. and W. represent the horizon; C, D, F, an azimuth circle, passing through the sun's centre; now an observer, placed at the centre, will see the sun at rising, in the line 1, but when he gets a greater altitude, and arrives at E, he will see the sun in the line and as the sun alters its altitude, will be seen in the lines O 3, 4, O 5, at length will arrive at its meridian, "Z, S, and the figures, 2, 3, 4, 5, will represent the different magnetic azimuths; the difference between any of these and the true azimuth found by calculation, is the variation.

EXAMPLE IV.

EXAMPLE V.

Suppose the sun's true amplitude Suppose the sun's true azimuth at rising is E. 13° 24′ N. and his in the forenoon is N. 86° 40′ easterly, magnetic amplitude E. 12° 32′ S. but by the compass it is N. 73° 24′ required the variation, and which easterly; required the variation, and way. which way.

Since the true amplitude and ob-
served have different names,
To the true amplitude E. 13°
Add the magnetic amp. E.12

Since the true and observed azimuths are both on the same side of

24'N. the meridian,
32 S. From the greater
Take the lesser

Their sum is the variation 25 56W.

Remainder variation

N. 86° 40′ E.

N. 73 24 E.

13 16 E.

Which is westerly, because the Which is easterly, because 1 rue amplitude is to the left of the true azimuth is to the right e bserved.

Jobserved.

E

Which is easterly, because the Which is west, because the true azimuth is to the right of the true azimuth is to the left of the observed. Jobserved.

The use of the variation is to correct the course steered by the compass; when the variation is east, it must be allowed to the right hand upon every course steered quite round the compass; but when the variation is west, to the left hand.

NOTE. The variation may be easily found by taking the sun's altitude in the morning, and observing what point of the compass he bears upon; and in the afternoon when the altitude is the same the middle point will be the true meridian, the difference between which at the north or south points of the compass is the variation. If the altitudes are taken at 5, 6, or 7 o'clock in the morning, you will have the same altitude at 5, 6, or 7 o'clock in the evening, being equally distant from noon.

The variation of the compass was first observed at London, in the year 1580, to be 11° 15' easterly; and in the year 1622, it was 60 E. still decreasing, and the needle approaching the true meridian, until it coincided with it in the year 1662: since that time the variation still continues at London to increase westerly, at the rate of about 11 or 12 minutes every year; and is at this time about 23° 30' westerly, and in the English channel about 28° 00' westerly but how far it will go that way, time and observations will probably be the only means to discover.

The variation at Paris in the year 1640, was 3o E. but in the year 1681 it was 2° 21' W. and is now about 22° 20' westerly, still continuing to go westerly.

In short, from observations made in different parts of the world, it appears, that in different places the variation differs, both as to its quantity and denomination, it being east in one place, and west in another; the true cause and theory of which has not yet been discovered, and therefore in long voyages it is absolutely necessary that the mariner should find the variation of the compass by observation as often as possible.

161

THE METHOD OF KEEPING A SHIP'S RECKONING OR JOURNAL AT SEA.

By keeping a Ship's Reckoning, or Journal, is meant keeping an account of the ship's way, that the mariner may be able at any time to ascertain the latitude and longitude the ship is in; it therefore should be the great concern of every person who takes upon them the navigating of ships to remote parts, to be expert therein, as the lives and fortunes of so many men are committed to their charge.

When a ship is bound from one place to another, which lies so far from her that she is obliged to go out of sight of land for any considerable time, as from England to Jamaica; at the time of her leaving sight of land, she is said to take her departure, and that part of the land she then leaves is said to be the place she takes her departure from; such as the Land's-end, Lizard, &c. and at the time of taking such departure, the captain or mate generally takes the bearing or distance of that land (according to his judgement,) and sets it down on the log-board, or in the log-book, against the time it was taken, thus, Land's-end, N. N. E. dist. 7 leagues; or Lizard N. by W. dist. 5 leagues, &c.

In the same manner may the departure from any place be taken, as may be seen in the first day's log. of the following journal, where the log-book is marked in columns for hours, knots, fathoms, courses, winds, lee-way, transactions; and under it the columns for courses, distances, northings, or southings, eastings, or westings, the latitude by dead reckoning, latitude by observation, meridian distance, difference of longitude, longitude in, and in the last, bearing and distance of the land.

Notice must be taken, that in the column for course, you are always to set down the course you have made by your reckoning for that twenty-four hours; that is, from the noon of the day before to the noon of the day you work on, the sea account being always kept from noon to noon.

Dead reckoning is that account deduced from occurrences which are written on the log-board.

In the columns for distance you are to set down the distance made by your reckoning for that twenty-four hours.

In the columns of northing and southing, you are to set down he difference of latitude made in that twenty-four hours, markng the column with north, if the difference of latitude be north, nd south, if south.

In the column of easting or westing, you are to set down the dearture made that twenty-four hours, marking the column with ast, if the departure be east, and with west, if westerly.

In the column marked latitude by D. R. you are to set down the atitude you reckon yourself in on that day; and in the column marked lat. by ob. you are to set down the latitude found by obse Y