Dia-magnetic.

Common air was also discovered to have a magnetic action, and hot air is more dia-magnetic than cold. Oxygen is as para-magnetic in the air as iron is on the earth, and this, it was considered, may give rise to magnetic storms, and account for the declination of the needle.

We may now proceed to consider the Mariner’s Compass. The compass, or the mariner’s compass, is so common that it is scarcely necessary to give a long description of it. Its history is unknown. The Chinese seem to have been aware of its usefulness long before the western nations adopted it. It was about the time of the Crusades that it was brought into western prominence, but was not generally known till the thirteenth century. Chinese writings ascribe to the compass a great antiquity; they maintain that it was discovered two thousand five hundred years before the Christian Era, and used for travelling on land. But, according to other accounts, it was not used at sea till the year 300 A.D.

The Chinese put the south first when speaking of the points of the compass, and in the Chinese empire and Thibet west goes likewise before east. So the imperial edifices in China face the south, and the needle, in their expression, points south and north—not as we say, north and south. The antiquity of the compass may be inferred from the recorded fact in Chinese chronicles, written in the second century before the Christian Era, that nine hundred years previously to the date of the chronicle the Emperor gave magnetic cars to certain ambassadors to guide them home in safety. These cars were fitted with a magnetic needle which communicated with a figure. Its outstretched hand and finger followed the compass-direction, and pointed out the way.

The Chinese subsequently (in the twelfth century) suspended the needle by a thread, and it is said their philosophers at that time noticed the variation of the needle. But Columbus first, in 1492, and Cabot, in 1540, certainly remarked it in Europe. It is to Marco Polo that we are indebted for the direct introduction of the needle into Europe, although it probably had been in use in the Levant previously, for we have seen a quotation by an Arab writer, who, in 1242, described the needle as being used at that time on his voyage from Tripoli in Syria to Alexandria, two years previously.

Friar Bacon possessed a loadstone, and there are many instances in which it is referred to in ancient writings. The inventor of the compass we cannot trace, but no doubt exists as to its being of Chinese origin.

The ordinary compass is shown in the illustration herewith (fig. 265). It consists of a magnetized needle, suspended freely, and fixed to a circular card, which is divided and subdivided into thirty-two points, as in the cut. This compass is suspended upon gimbals to keep it in an upright position when the vessel rolls or plunges. The gimbals are concentric rings, the compass being fastened to the inner one, and keeps its position in all weathers. It is then enclosed in the binnacle, a glass receptacle. The card moves with the needle which points north. There is a dark line (lubber line) which indicates the ship’s course, and when sailing the steersman must keep that line opposite the compass direction-point which indicates the course. At night a lamp is lighted in the binnacle, and the card being transparent and the points opaque they are easily seen.

The magnetism of iron ships has a tendency to disturb the needle, and many suggestions have been made and discussed with a view to obviate this. To put the compass at the mast-head was one, to surround the compass with “counter-irritants” another. But the usual way is to “swing the ship,” and so adjust the compass. Swinging the ship means turning her round point by point, and marking the deflection of the needle with reference to a certain object. The amount of deflection at each point is read and noted, and subsequently taken into consideration when sailing.

The Azimuth Compass is a mariner’s compass fitted with brass uprights slit through the centre, through which the heavenly bodies may be seen. These are the sights. The card is divided into degrees and quarters. A fine wire is fixed upon one of the sights, and in the other slit is a prism to reflect the divisions of the card to the eye. The object—the azimuth distance of which it is desirable to know—is looked at through the slit, and bisected by the wire. The divisions of the scale are at the same time reflected, and the number read gives the azimuth distance required.