field, the procedure remains to be illustrated; and this will now be done by an experiment with the SCORESBY.

It will be recalled that Fig. 553-without the masses marked E and F, however—represents the SCORESBY loaded with the disturbing material that produced the primary field that caused the deviations of Table 93: these having been compensated, the residuals of Table 95 remained. Then produced a secondary disturbing field by placing at E, Fig. 553, two bars of soft iron, each 18 inches long and about 2 inches cross-section; they were horizontal, in the midship line, on a level with the compass, and nearest ends 10 inches from pivot: also placed at F, parallel to D, eleven cylindrical steel magnets, each 10 inches long, with their nearest ends 32 inches from pivot.

Then swung ship and found the deviations of col. (2), Table 99: analyzed this, and the resulting coefficients are given in cols. (5) and (6), Table 100.

The algebraic sum of the values of B and C in cols. (4)

C' +.5564,

and (6) gives B' and C' col. (8); then tan a = B'

+.3232 whence a = 59° 51'; and this is the new starboard angle to which the tray (for the resultant method) must be turned.

of p; the needle will incline, and to restore level a
steel magnet must be placed centrally beneath the c
then move the bead from its last position by the amo
the needle will incline again, but this time level
restored by moving the Flinders' bar. Thus the la
be used for correcting the vertical effect of soft in
the semicircular deviation and the heeling error.

COMPENSATION OF THE COMPASS.

Should the ship at any time be on the Magnetin
where there is no vertical induction, the part of
error due to hard iron can be corrected as follow
ship E. or W. magnetic and have her upright
compass by the balanced needle, without the b
dip under the influence of the vertical compor
ship's permanent magnetism, and must be rest
by a vertical steel magnet placed centrally bene
pass. Subsequently, upon arrival in a local
Dip is large, the Earth's vertical componer
polarity in the vertical soft iron and cause
needle-placed as before-to incline: it mu
to level by moving the wrought-iron corrector
bar; and thus the heeling error has been
appropriately corrected.

Aside from all the foregoing, change is
in the permanent magnetism of the ship and
-it is but the fate of all magnetic bodies, e
steel; again, new masses of soft iron may
the compass to influence it; or, finally, e
the many causes enumerated in Art. 25
break the adjustment: the result in all su
series of deviations.

If it is decided to remove the correcte the deviations due to both the primary an and compensate them de novo, the proces already been explained: but if it be pre correctors in place, and simply neutralize

Section Five: Summary.

385. The complete magnetic examination of a ship.-The individual parts of this examination have been described in various places in this Treatise-they will now be gathered under one head.

As soon as the ship has taken shape, observations may be conducted for deviation and horizontal and vertical force in all the possible sites for the compasses; and if continued at intervals while she is on the stocks they constitute the introductory chapter of her magnetic history.

Such information will enable the observer to decide, by the date of launching, upon the locality (within narrow limits) for each compass.

When the ship is completed and in dry dock, the magnetic survey should be made; it will determine the distribution of magnetism in the hull and indicate the neutral line: this found, the compasses should be placed as near it as circumstances will permit.

The exact spot for each compass should be determined while the ship is in dock by a series of observations throughout the region in view for their location: such an investigation is described in Art. 266.

The binnacles are then set up and the compasses installed.

Very careful observations of deviation and horizontal and vertical force should now be made at the place of the standard compass; a similar series is to be made at the same place when the ship comes out of dock and is put on a diametrically opposite heading alongside a wharf free from iron either in mass or in other ships; a third series is to be made in a spot on shore free from iron: these three series together afford the means of obtaining a Table of Deviations, of calculating the Heeling Error, and of Compensat

ing the Compass-all approximately, of course, as explained in Part Fourth; but which it may sometimes be necessary to have recourse to until accurate methods can be followed.

As the occasion of the ship being in dock or quiet at a wharf affords the best opportunity she probably may have for observations of force, horizontal and vertical, they should be made with extreme accuracy and fullness: they yield data of great value subsequently.

Presuming that there are Compass-buoys, and that the ship can be taken to them, this should be done—in addition to the foregoing. Swinging at buoys-with each helmupright—and heeled, if possible—is the most accurate way of determining the deviations. While on the four principal points during the swinging, if the water be smooth and the weather calm, observations for force, both horizontal and vertical, might be carried on: they are most desirable and may prove quite reliable. After deviation tables are obtained and analyzed, the compasses can be compensated.

Should buoys not exist, the swinging may be done by steaming in a circle in smooth water and the compensation performed at the same time.

When the vessel proceeds on her cruise, and reaches a port where the Earth's magnetic elements differ considerably from what they were at the port of sailing, observations should be made for correcting the effect of vertical soft iron.

With all the foregoing thoroughly performed, the magnetic character of the ship stands revealed and the navigator is in a position to deal intelligently with its every change— just as he would be to humor the moods and eccentricities of a person with whom he was intimately acquainted.

On the other hand, if he possess only the meager knowledge of a single rapid swing on sixteen points, he will be as much in doubt and perplexity as to his action in any contingency as in his intercourse with a stranger of whom he knew only the name-and that an obscure one.

CHAPTER XXX.

386. Scope, character, and aim of this Treatise. Much of the course at the U. S. Naval Academy is occupied with mathematics-and justly so; for aside from its mental training, it adequately equips the midshipman for the performance of many duties in his subsequent career.

Pursued through the sequence of nautical astronomy and navigation, the course produces the navigator-to cite only one part of the profession-who comprehends the dependence to be placed on every method for finding the Ship's position at sea-when to take advantage of favorable conditions-when to avoid those fraught with inaccuracywhen the problem is entirely practicable-and when it is wholly impossible.

And this is the educated navigator that the costly seastructures of to-day should have.

An essential part of Navigation is the subject-matter of this Treatise; indeed it would seem a most important part; for whereas many means exist for finding the ship's position, only one is known for guiding her—the Compass.

But that this instrument itself is beset by much to lead it astray is amply shown in the foregoing pages; and hence it is incumbent on the navigator to inform himself thoroughly on its every source of error-to gain that intelligent grasp of the conditions midst which it is placed that the course otherwise in Navigation affords for finding the ship's position.