one is N. and the other S., take their sum; the result in each case will be the Error of Compass.

Then, looking from the centre of the compass in the direction of the observed Amplitude—name Error E. if true, is to right of observed, Amplitude;. but name Error W. if true, is to left of observed, Amplitude.

Note. The observed Amplitude is of course the object's bearing by compass at rising or setting. If you have no deviation, the Err. of Compass is the Variation.

3. For the True Azimuth, you will find that in the works to which I have referred (p. 93), Burdwood, Davis, and Labrosse, the bearing of the object is named from N. in N. Latitude, but from S. in S. Lat., towards E. before it comes to the meridian, but towards W. after it has passed the meridian. Hence if true and observed Az. have different names, one N. and the other S., they can be brought into the same name by subtracting one from 180°; thus S. 110° E.-N. 70° E.

4. For the Error of the Compass.—Under the True Azimuth write the observed Azimuth or Bearing, both reckoned from the same point, N. or S.; then—

If both are E., or both W., take their difference; if one is E. and the other W., take their sum; the result in each case will be the Error of Compass.

Then, looking from the centre of the compass in the direction of the observed Azimuth—name Error E. if true, is to right of observed, Azimuth; but name Error W. if true, is to left of observed, Azimuth.

Thus far, whether by Amplitude or Azimuth, you know how to find the Error of the Compass; but we must go a step further.

5. For the Deviation of the compass.-Under the Error of Compass write the Variation as given on Chart; then with Error and Var., both E. or both W., take their difference ;—but with Error and Iran, one E. and the other W., take their sum.

The resulting Deviation will be of the same name as the Error of Compass; unless the Error has been subtracted from the Variation, in which case the Deviation will be E. when Error is W., but W. when Error is E.

Also, with Error 0, Deviation is of the same amount as the Variation, but of the opposite name; with Variation 0, the Error is the Deviation.

Remember that the Deviation thus obtained is that due to the course on which the ship's head may be at the time of making the observation.

By book, T. Bearing N. 88° E.

Ex. On June 1st you are in Lat. 48° N., Long. 34° W.; the ship is heading N.W. W. by compass; at about 7h. 15m. A.M. you take the Sun's Bearing by Compass as S. 79° E.; you then see from the Naut. Alm. that the Sun's Dec. is about 22° N.; you turn to the given Lat., and for the corresponding Dec. and Time, you find the True Bearing N. 88° E.; you then work the Ex. as at the side, and find that the Dev. with the ship's head N.W. W.

S. 92 E.

Obs.

S. 79 E.

Err. of Comp.
Var. by Chart.

13 W.

35 W.

Deviation

22 E.

is 22° E.; and that since the Err. of the Comp. on that point is 13° W., the course made good will be N. 633° W. or N.W. by W. W.

This is a Time Azimuth, which requires no observed altitude; and if you change your course twenty times

a day you can always, in a few minutes, know the exact course you are making.

Ex. September 2nd; Lat. 39° N.; Long. 48° W.; at 3h. 45m. P.M. app. time at ship; sun bore by compass N. 80° W.; ship head

ing by compass South; sun's Dec. 8° N.; sun's true bearing N. 106° W.; Variation 23° W.

Ans. Err. of Comp. 26° W.; Dev. 3° W.; True Course S. 26° E.

Ex. October 3rd; Lat. 50° N.; Long. 15° W.; at 8h. A.м. app. time at ship; sun bore by compass S. 28° E.; ship heading by compass West; sun's Dec. 4° S.; sun's true bearing N. 116° E.; Variation 27° W.

Ans. Err. of Comp. 36° W.; Dev. 9° W.; True Course S. 54° W. Ex. May 16th; Lat. 61° N.; Long. 22° W.; at 4h. 32m. A.M. app. time; ship heading by compass S.E.; sun bore by compass N. 60° E.; sun's Dec. 19° N.; sun's true bearing N. 62° E.; Variation 42° W.

Ans. Err. of Comp. 2° E.; Dev. 44° E.; True Course S. 43° E. Ex. August 24th; Lat. 49° N.; Long. 30° W.; sun rose by compass East; ship heading by compass N. by E. E.; Variation 35° W.; sun's true Amplitude E. 17° N.

Ans. Err. of Comp. 17° W.; Dev. 18° E.; True Course North (nearly). Ex. August 1st; Lat. 50° N.; Long. 15° W.; sun set by compass N. 21° W.; ship heading by compass N. E.; Variation 27° W.; sun's true Amplitude W. 29° N.

Ans. Err. of Comp. 40° W.; Dev. 13° W.; True Course N. 37° W.

Note. For a Time-Azimuth use apparent time.

GREAT CIRCLE SAILING.

The earliest works on Navigation treat on Great Circle Sailing, and many of the early navigators, when traversing the Atlantic and Pacific, availed themselves of its principles when they found that they could do so with advantage. But land, ice, wind and weather, current, or other causes, considerably limit the adoption of this method of sailing.

Mercator and Parallel Sailing conduct the ship by a circuitous route, when compared with the track of a Great Circle; but the simplicity of the calculations connected with these modes of sailing, and the circumstance that by compass the ship's track showed but one course from port to port, led to the adoption of the two systems above named in preference to Great Circle Sailing, in which the course is continually changing.

The following elucidations, from Lieutenant Raper's valuable "Practice of Navigation," present, in various points of view, the difference between Mercator Sailing and Great Circle Sailing. By Mercator Sailing "the ship crosses at the same angle all the meridians which she passes over; that is, her head is kept on the same point of the compass until she reaches her intended port; and in this consists the convenience of these sailings. The track of a ship thus steered is, however, very different from the shortest distance between any two

F

places, or the distance as the crow flies; the sailing by which the direct course to a place is to be shaped is called Great Circle Sailing."

"The course on the rhumb-line from one of two places to the other is exactly the opposite of the course to that place from the other; while on the Great Circle these courses are very different. The ship, while on the rhumb-line, is always changing the direction of her head with respect to her port, for which she never steers exactly until it is in sight, because this track cuts all the meridians at the same angle, and the meridians themselves are not parallel to each other; but on a great circle she steers directly for her port, while the angle made by her track is perpetually varying."

"The track on the great circle and that on the rhumb-line differ most widely from each other in high latitudes, and between places on nearly the same parallels.

When the ship sails on a great circle between two places, without crossing the equator, she is always in a higher latitude than if she had sailed by a rhumbline hence, since both tracks coincide at their extremities, there must be a point in the great circle at which the distance, measured on a meridian to the rhumb-line, is greater than anywhere else. When the ship crosses the equator, there are two such points, the one being to the northward of the rhumb-line in north latitude, and the other to the southward of the rhumbline in south latitude."

To determine, by calculation, how much the Great Circle course must be changed from time to time, is a very tedious process, and one little likely to be welcome to the navigator. Unless the results could be tabulated, so as to leave nothing but a few easy rules to observe, Great Circle Sailing would ever be an exceptionable system: now this is precisely the nature