a distance that the space through which the ship revolves shall make no sensible difference in its bearing. The ship is gradually swung round with her head successively upon each of the 32 points of the compass and the bearing of the object taken. The true magnetic bearing of the object taken from the ship is next determined, which is effected by taking the compass to some place on shore from whence the part of the ship where the compass stood and the object of which the bearings. had been observed shall be in one with the observer's eye, or else in the exactly opposite direction. The bearing of the object from that spot will evidently be the true magnetic bearing from the ship by the compass. The difference between the true magnetic bearing of the object, and the successive bearings which were observed with the compass on board, when the ship's head was on the several points, will show the error at each of those points which was caused by the ship's iron.

EXAMPLE.

When the ship's head lies N.N.E., let the binnacle compass bearing of the shore object or compass be N. 19° 30′ E., and the bearing of the binnacle compass from the shore compass be S. 27° 0' W.: required the deviation.

The opposite point to S. 27° 0' W. is N. 27° 0' E., which is 7° 30′ to the right of N. 19° 30' E. Hence the deviation is 7° 30′ E.

If during the operation of swinging, a haze obscures the shore compass, while the sun at the time is shining brightly, a number of points may be secured by time-azimuths, which otherwise might be lost. Timeazimuths are also advantageous where the second of the above methods cannot be used for want of an assistant observer for the shore compass; and when the first of the above methods are not available owing to the length of the ship (e.g. the Great Eastern) and the scope of the moorings, combined with the most distant objects in sight, not being sufficiently far off to render the difference of their bearings insensible as the ship swings round to the tide. In such cases Godfray's Azimuth Diagram will be found useful.

The following are the leading principles which have been developed in the course of repeated investigations of the magnetism of iron with reference to the compass-action and its changes in iron ships, viz. :

1. That the magnetism of iron ships, in its action on the compass, may be represented by a vertical and horizontal iron or magnetic bar swinging round the compass. 2. That changes take place in the magnetic distribution and compass-action in iron ships. 3. That the changes take place in a ship's magnetism by changes of magnetic latitude. 4. That there are influences in a ship derived from the varieties of form and position (relatively to the compass) of particular masses of iron, which may act

as natural correctives. 5. That the plan of correcting the deviation of iron ships, either by a plate of soft iron, as proposed long ago by Mr. Barlow, or by a magnet or two magnets, as proposed by the present Astronomer-Royal (unless for limited voyages, as in the case of vessels plying between ports in the United Kingdom, or even in Europe, or plying between England and the United States), is unsafe, and in going to southern latitudes aggravates the error. 6. That the twisting and straining of the iron materials of a ship will tend, especially in ships recently launched, to alter the magnetic action on the compass. 7. That it requires time to offect the changes in a ships magnetic distribution, which ultimately may, in regions distant from the place of building, be effected.

Under the following circumstances great changes may be expected :

:

1. In new ships first encountering heavy straining or rolling by sea. 2. In ships generally, if following a new voyage. 3. In ships running long on one course, and then changing the course. 4. Heavy weather first occurring.

No changes of importance may be expected under the following circumstances :

1. In iron vessels long in use, and ordinarily pursuing the same voyage, because extreme deviation gets shaken down, as it were, in a medium or average state. 2. Great changes do not take place in the retentive magnetism, which is by far the greatest portion in latitudes not further south than the Mediterranean, because-3. In ships trading in the Channels (British, St. George's, &c.), or east and west to America, the liability to new and unexpected changes greatly diminishes.

Dr. Scoresby's suggestions for diminishing the dangers arising from the deviation of the compass

are:

1. A standard azimuth compass, to be placed on a high pedestal, where (on the Admiralty plan) a position of small deviation may be found. 2. A compass fixed at the mast-head (fitting the mast-head with brass work instead of iron work) for reference will, he believes, be best of all. This method, however, has not been sufficiently tried to be recommended for general adoption. 3. The wheelcompass required for ships engaged in the home trade, or traversing mainly parallels of latitude not southward of the Mediterranean, if adjusted with magnets and pieces of iron, may not then be unsafe, where reference may always be had to the standard for verification. 4. No standard compass in great distances. 5. Care to be taken, in the selection of compasses, that they have ample directive force. 6. Captains must, at every opportunity, take observations for the verification of their compass, by azimuths, stars, position of the land, &c. 7. Captains should have special knowledge for the charge of iron ships, because in this case, in addition to the ordinary dangers of navigation, there is a new source of error and misguidance, with respect to which it is most important they should never be thrown off their guard.

The plan adopted by the Royal Navy (which is preferable) of determining the amount of deviation, is by placing the ship's head on a number of compass points as nearly equidistant as possible, and the deviation on each point is observed, either by comparison with a compass, or by the bearing of a distant terrestrial object or of a heavenly body. The method of doing so by comparison with a compass is described above.

When the deviations have been observed on a few points only, it is requisite to compute the deviation on the intermediate points; and when from deviations on a large number of points, to compute the most probable value of the deviation on each point. The solution of these two problems may be effected mathematically by computing the co-efficients, ABCDE, from the observation by the method of least squares; and when the observatons are made on equidistant points the calculations are very simple. For the method of doing this see Supplement to the Practical Rules for ascertaining or applying the Deviation of the Compass caused by the Iron in a Ship.

For further information on this important subject the reader is referred to "The Practice of Navigation," by Lieut. Raper, R.N., pp. 69-80; "Practical Illustrations of the Necessity for Ascertaining the Deviation of the Compass," by the late Capt. E. Johnson, R.N., F.R.S.; "The Magnetism of Iron Ships and the Mariner's Compass," by Commander Walker; "Admiralty Manual for ascertaining and applying the Deviation of the Compass," by F. J. Evans, Master, R.N., F.R.S., and Archibald Smith, M.A., F.R.S.; also the "Practical Rules," by A. Smith, Esq., published by the Admiralty, containing a description of another method, called the Graphic Method, which is the most convenient one in ordinary cases, and proposed in different shapes by Mr. J. N. Napier, of Glasgow, and Capt. Ryder, R.N. The two methods are the same in principle. That of Mr. Napier will perhaps be found more convenient in construction by the expert; Capt. Ryder's simpler in use by the inexpert. When the deviation is corrected, either by a table or by the Graphic Method, it must be remembered that the same table or the same curve can only be used in the same magnetic latitude, and as long as there is no material change in arrangement or condition of the iron of the ship. Whenever any considerable change in the magnetic latitude of the ship takes place, a fresh table should be formed, and a fresh curve constructed, from fresh observations.

The following is a Table of Deviations to which reference will be made hereafter. The Table is copied from the sheets issued by the Naval Department of the Board of Trade, for the use of candidates under examination ::

To find the correct magnetic course, having given the compass course and deviation.

Easterly deviation allow to the right.

Correct the following courses steered as given in the Table, page 73.

In all collections of tables for the use of navigators there is inserted a table containing the true difference of latitude and departure, corresponding to certain distances (at intervals of one mile) up to 300 nautical miles, for every course, at intervals of a quarter point, and also of degrees, from o to a right angle (90°). Tables I and II (Raper or Norie).

In these Tables the course is found at the top of the table, when under four points or 45°; but at the bottom of the table, when it exceeds four points or 45°. The first column contains the distance to 60 miles, the second column contains the difference of latitude, expressed in minutes and tenths, and the third column, similarly expressed, contains the departure; but if the course exceeds four points or 45°, the second column contains the departure, and the third column the difference of latitude. The other columns are a continuation of the former, exactly upon the same principle, and extending to 300 miles of distance.* (See Tables I and II, Norie and Raper.)

USE OF THE TABLE.

Given the course and distance, to find the difference of latitude and departure.

EXAMPLES.

Ex. I. A ship sails N.W. N. a distance of 78 miles: required the difference of latitude and departure by inspection.

The given course is 3 points; and referring to Table I, we find the page devoted to this course to be page 14, Norie, or page 436, Raper's Navigation, in which against 78, in column headed Dist., stands 60'3 under the head Lat., and 49°5 under the head Dep. We conclude therefore that, for the given course and distance, the difference of latitude is 60 3 miles, and the departure 49'5 miles.

Ex. 2. Suppose the course to be 5 points, and the distance 98 miles.

Then, since the course here exceeds four points, we look for it at the foot of the page, (page 10, Norie, or 432, Raper) and against 98 in the distance column we find 86.4 in the adjacent departure column, and 46°2 in the difference of latitude column, so that the difference of latitude made is 46°2, and the departure 86.4.

Ex. 3. Course N.E. by N., distance 129 miles: find diff. lat, and dep.

Enter Table I, and find 3 points at the top, and in one of the columns marked Dist, find the distance 129, then in the columns opposite to this, marked lat. and dep., stands the difference of latitude 107'3, and departure 71.7.

* This table is constructed by solving a right-angled triangle, of which one angle represents the course, and the hypothenuse the distance; by giving these different and successive values, the corresponding values of the other two sides are found, which sides represent the true difference of latitude and departure. It is evident that the difference of latitude and departure for any course, are the departure and difference of latitude for the complement of that course, and hence the table is compactly arranged by interchanging the headings of the columns containing these elements at the top and at the bottom of the page, and using the top reading for courses from o° to 45°, and the bottom reading for courses from 45° to 90°. This table may be used for a vast number of problems, depending for their solution on the relation of the several parts of a right-angled triangle, and, since all the relations between any two quantities may be expressed as functions of some angle, in terms of of the sine, cosine, or tangent, it may be used, in fact, as a general proportional table.