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.