77.

Reversion or destruction of the magnetism. Origin of the term 'Subpermanent.'

Suppose that the lower end of the bar in the experiment of Article 75 is distinguished by being painted white. This white end, after the bar has been placed with its white end in the direction of dip and has been struck, is found to be charged with red magnetism. Now reverse the bar upon the dip plane with its white end upwards, and strike it; it will be found that its white end is charged with blue magnetism. The magnetism has been reversed. Undoubtedly, in order to arrive at this state, it has gone through the stage of being destroyed or rendered undiscoverable by instruments.

But the pure destruction may be visibly effected in the following manner. Lay the bar upon that surface of the magnetic anvil which is normal to the direction of dip, and strike it with a few blows of the hammer. On removing the bar, and testing its state by means of a compass, it will be found that all trace of magnetism has disappeared. The bar is now in the same state as before the experiment of Article 75.

If, however, the magnetized bar be subject to no such violence, but be suffered to rest quietly, or be moved gently into different positions, it will slowly lose a large proportion of its magnetism. And it is this peculiar character which necessitated the introduction of a new name. The magnetism of a struck iron bar resembles the magnetism of a permanent

steel magnet in all respects but this, that, while perhaps no change can be remarked in hours or days, it infallibly diminishes in a long time. To express this partially permanent character, the term 'Subpermanent Magnetism' has been adopted.

In single bars, the subpermanent magnetism diminishes sensibly in a few hours, and is lost in a few days. In some large iron ships, a portion of it has remained unaltered for many years. It would seem that where the operation of magnetizing by hammerblows has been rapid, the magnetism is not very firmly fixed: but where the violence has been long continued, the magnetism is so firmly established as to become an immovable quality of the iron.

SECTION X.

ON THE MAGNETISM OF IRON SHIPS, AS AFFECTING THEIR COMPASSES.

78. Philosophical and Commercial Importance of this subject. Complication of the Magnetic considerations involved in it.

It is unnecessary to remark on the extent to which at the present time, when so large a portion even of the mercantile navies is built of iron, the interests of commerce are involved in the investigations which alone can make the ship-compasses available. But it may be desirable to point out to what an extent Science may benefit from it. It will be shewn that the principal agent in the disturbance of the compass is subpermanent magnetism, an element little known before the introduction of iron ships, and whose laws have principally been derived from the examination of iron ships. But another element, whose effects are sensible in all, and very important in some, is transient

induced magnetism: and the study which in late years has been given to this subject has been stimulated almost entirely by its application to iron ships.

In applying the science deductively to the control of ships' compasses, every part of the theories treated in the earlier sections is brought into play. Magnetic declination is obviously necessary: terrestrial horizontal force enters into every formula of disturbance (see Article 55): both horizontal force and vertical force, or dip, occur in the formulæ for induction disturbance: the laws of action of magnet-power enter both into the effects of subpermanent magnetism of the ship and into those of the permanent magnets employed in correction: and the theory of induced magnetism, and especially of quadrantal deviation, presents itself in the correction of the effects of the iron masses.

79. Brief history of the first steps in this science.

The first real step appears to have been made by Captain Flinders, about 1803, who remarked that the disturbances of his compass were such as would be produced by the attraction of iron charged with magnetism; blue for northern latitudes and red for southern latitudes, in the direction of the ship's head; and suggested the use of a vertical bar to be placed aft of the compass, whose upper end having similar magnetism would tend to correct the other. At a later time, 1820 to 1833, numerous experiments were made by Professors Barlow and Christie, illustrating the action of induced

magnetism. In regard to subpermanent magnetism, the first experiments on iron bars, &c., were made by Mr Scoresby, about 1821; and the first virtual observation in ships was made by General Sabine in discussing the compass-deviations in Sir James Ross's voyage, 1839 to 1843, in which he remarked that the peculiarities in the disturbances of the compass lasted for a short time after the ship had left the region in which the terrestrial forces were such as would tend to explain the disturbances. These observations were made in wood-built ships having many accidental masses of

iron.

The first explanation of the character of the compass-disturbance produced by iron ships was given by the writer of this Treatise in the Phil. Trans., 1839, as resulting principally from examination of the iron steamer, Rainbow, in 1838. The disturbing forces on the steering-compass of that ship were so great that in one position of the ship the north end of the needle was deflected more than 50° to the east, and in another position it was more than 50° to the west. The first light that was thrown upon the causes of these deviations was obtained by placing the ship with her head exactly north (which can be done in various ways, one of the most convenient being to use an azimuth-compass on shore, and to adjust the ship by signal till her masts, as seen by the shore-compass, are all in the magnetic meridian), then observing the deviation of the compass, and, replacing the compass by a vibrating needle whose time of vibration on shore