can be had. If when the ship heels steadily over to either side, the North point of the compass shows a deviation towards the upper side of the ship, the HEELING MAGNET if red end up is to be moved up a little, or if blue end up to be moved down. If, on the contrary, the North point of the compass shows a deviation towards the lower side of the ship, the HEELING MAGNET must be moved downwards if its red end is up, or upwards if its blue end is up. If the ship crosses the magnetic equator,

it

may be found necessary to reverse the HEELING MAGNET. A Clinometer (little pendulum) attached to the binnacle facilitates this adjustment, by allowing the amount of every variation of the ship's heeling inclination to be seen along with the deviation of the compass produced by it.

To understand the action of the Flinders-bar, suppose the ship to be anywhere in the northern magnetic hemisphere, the vertical force there is such as to pull the red end or pole of a magnetized needle downwards, and to repel the blue end upwards. It also has the effect of inducing magnetism in any mass of iron, so as to give it a transient magnetic quality marked with blue on the upper side or end, and red on the lower side or end. Thus, in the northern magnetic hemisphere, the Flindersbar is transiently magnetized by the earth's vertical force in such manner that it acts like a great bar magnet, with its end blue and its lower end red. At the upper At the magnetic equator it loses its magnetism, and in the southern magnetic hemisphere it acquires magnetism in the opposite direction to that which it had in the northern hemisphere; so that its upper end becomes red and its lower end blue. As the ship moves from one hemisphere across the magnetic equator to the other, the magnetism of the Flindersbar gradually diminishes to zero, and then increases gradually in the contrary direction.

The object to be attained in applying it to the binnacle is, that, with this gradual change of its magnetism, it

shall always as exactly as possible counterbalance the changing part of the force on the compass, due to the part of the ship's magnetization which changes with the gradual change of a vertical component of the terrestrial magnetic force. If this changing part of the ship's disturbing force on the compass is a pull aft in the northern magnetic hemisphere, and a pull forward in the southern magnetic hemisphere, the Flinders-bar must be on the forward side of the binnacle. On the other hand, if the regularly changing part of the ship's force be a pull forward in the northern hemisphere, and aft in the southern hemisphere, the Flinders-bar must be on the after side of the binnacle.

The former is the most frequent case for the Standard compass and the steering compasses of modern merchant steamers in which the conning and steering of the ship is done on a bridge forward of the engines, with considerably more than half the ship behind it. It is almost certain to be the case with an after steering compass, a few feet in advance of the top of the stern post and rudder head, in an iron steamer or sailing ship. The second above-mentioned case is what will generally be found with a compass placed anywhere in the after half of the ship's length, to within six or eight feet of the stern post. Generally it is impossible to ascertain which of the two is the actual case until the ship has made a voyage through regions presenting considerable differences of vertical magnetic force.

The best plan generally is, in first placing the binnacle on the deck, to turn it with the fittings of the Flindersbar forward or aft, according as it is found that the foreand-aft correcting magnets have to be placed with red or blue poles forward or aft. It may be that the experience of a first voyage may show that the binnacle must be turned the other way to get the Flinders-bar into its right position, but the chances of this being necessary are

reduced if the binnacle is originally placed according to the preceding rule.

In the first adjustment, or when there is knowledge as to the proper proportion of correction to be made by the Flinders-bar, it may be set to correct about half the error on East and West courses; the remainder must be accurately corrected by the fore-and-aft magnets.

Suppose now the first adjustment to have been made. somewhere in the northern magnetic hemisphere, and suppose that as the ship goes to places of weaker vertical force the fore-and-aft correcting force required to make the compass correct on East and West courses is found to be less than at the beginning of the voyage, then it is clear that part of the correction made by the magnets ought to have been made by the Flinders-bar. But nothing need be done except to diminish the fore-and-aft pull by the magnets, as long as the ship is going to places of weaker vertical force. If without touching or crossing the magnetic equator the ship returns again to places of stronger vertical force, and if it is found that increased longitudinal pull is now required, this should be applied, not by the magnets, but by introducing a Flinders-bar or by increasing the bar already in position.

Generally, for a ship making passages to and fro through regions of considerably different vertical force, whether she crosses the magnetic equator or not, the rule in respect of the fore-and-aft correction is as follows:

CORRECT THE DEVIATIONS FOUND BY OBSERVATION ON THE EAST OR WEST COURSES BY THE FORE-AND-AFT MAGNETS WHEN THE SHIP IS GOING TO PLACES OF WEAKER VERTICAL

FORCE; AND BY THE FLINDERS-BAR WHEN SHE IS GOING TO PLACES OF STRONGER VERTICAL FORCE, WHETHER IN THE NORTHERN OR SOUTHERN HEMISPHERE.

DEVIATION OF THE COMPASS IN IRON SHIPS.

Deviation is the amount the compass needle is drawn out of the magnetic meridian by the attraction of the iron in the ship. It is said to be East, when the north point of the compass card is drawn to the eastward or right, and West, when it is drawn to the westward or left of the magnetic meridian.

The terms East and West deviation are applied in precisely the same manner as East and West variation, that is, looking from the centre of the compass, apply East deviation to the right and West deviation to the left of the magnetic course to get the course to steer, and vice versa.

Deviation is determined, when in port, by finding the difference between the correct magnetic bearing of some distant object, and its bearing from on board. The object should be at such a distance from

the ship, say from 6 to 10 miles, that the diameter of the space through which she revolves shall w make no sensible difference in the real bearing of the object.

The

deviation thus obtained is said to

S11.15 W

N

-E

be Easterly when the correct magnetic bearing is to the right, and Westerly when to the left of the bearing from on board.

In the event of no suitable object being available, deviation is determined by reciprocal bearings, that is,

by landing a compass in a place free from the influence of volcanic rock or large masses of iron, and observing the mutual bearings of the shore and standard compasses from each other, at the moment when the ship's head is steady on each point successively.

The difference between the shore bearing reversed, and the standard compass bearing, is the deviation.

Deviation is determined at sea, by finding the true bearing of some celestial body by Azimuths or Amplitudes; the variation on the chart applied to the true bearing so determined, will give the correct magnetic bearing of the body, the difference between which and its bearing from on board will be the deviation for that direction of the ship's head.

When the ship's head is brought to one point, the deviation for that point only can be determined, but if the ship's head is brought to a number of points, the deviations for all the points of the compass may be determined.

For accuracy, the ship's head should be brought to at least eight points equally distributed round the compass.

When sailing along a well known coast, the deviation may be found by observing the bearing of two well defined objects in one straight line from the standard compass, the difference between which and their bearing from one another on the chart will be the deviation.

The deviation changes in any considerable change in the latitude of the ship, it changes for some time in a newly built ship, or, if the ship has been lying for any length of time with her head in one direction and then has her position altered, it also changes as the ship heels to starboard or port.

It is advisable to test the table of deviations at every available opportunity.

The heeling error is greatest when the ship's head is magnetic north or south, it vanishes when the ship's head is magnetic east or west.