ever way the vessel may lurch, the card is always in an horizontal position, and certain to point the true direction of the head of the ship. The concentric circles, or hoops, are termed gimbals or gymbals, and they are generally allowed to have been the invention of an Englishman, though there appears to be no evidence of the fact.

By whom the marking of the points was introduced is not known. The French have laid claim to the invention, and some of their authors have asserted that the marking of the four cardinal points was merely a modification of their fleur-delis; but a contrary question has been raised on the other hand, and a supposition has been started that the fleur-de-lis itself is only a modification of the mouasala, or dart, which the Arabs used to denote the direction of the needle, and which is employed to point out the north on our maps at the present time. Chaucer, who died in 1400, mentions the compass; and states, that the sailors reckon thirty-two points of the horizon, which is the present division of the card.

The discovery of the "declination of the needle," or that deviation from the true north and south which its poles evince, has been ascribed to an Englishman. It is unlikely that it could remain unknown long after the compass had been used as a nautical instrument in high latitudes. That Columbus was acquainted with it is evident from a passage in his life written by his son; and, in all probability, it proved one of the greatest difficulties with which he had to contend, especially as it is now known to vary in the different parts of the world, and is of necessity influenced by "terrestial magnetism," or the magnetism of the earth.

The words "terrestial magnetism" lead us, at once, to the most absorbing scientific question of the present day; but we must use the utmost brevity in touching upon it. What was called the "igneous theory," or doctrine of a central fire within the earth, has now given way to the belief, among philosophers,

that changes in the temperature of the air, and various other natural phenomena, are attributable to the earth's magnetic power. Professor Faraday, among Englishmen, has substantiated the grand fact, that the earth is one vast magnet; and Gauss, of Gottingen, computes the magnetic power of each cubic yard of the earth to be equal to that of six steel magnets, each of one pound weight.

It is also ascertained that magnetic currents are continually passing from the south to the north pole, through and around the earth. All the phenomena attributed to fire may be produced by these magnetic currents, while it would be difficult to admit the existence of interior fires unsupplied with the oxygen of the atmosphere. Now, not only are the causes of earthquakes, and of the action of volcanoes, rendered explicable by these discoveries; but the establishment of the fact that the electric currents are perpetually passing from the south to the north pole, through and around the earth, strips the "dip," or declination of the needle" of the mystery it has so long worn.

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In consequence of these influences there is a natural depression at that end of a magnetic needle, when it is suspended on its pivot, towards which the current of magnetism, as it may be called, is driven. This has been termed the dip; and many elaborate and careful experiments have been tried to ascertain precisely the amount of this dip; and through the observations made during these experiments, it has been discovered that it varies, and that a magnetic needle oscillates, to a certain extent, every twenty-four hours. In order to avoid the mischief that might arise if this were not allowed for, the magnetic needle of the mariner's compass is always suspended out of the mechanical centre of gravity.

If a needle, or other magnetized substance, be fixed on the top of a piece of cork, which is then placed on the surface of water, and left to float unrestrained, it will be found that one end of the needle will turn till it points nearly towards the

north. This is the point at which the current enters the needle; the other end will of course point nearly towards the south; and if the cork be turned round, so as to direct the needle to the points opposite to those towards which it was naturally directed, it will, as soon as it is released from compulsion, again assume the position which it previously held. This at once explains the reason why the mariner can direct his ship across the waves, even in the darkest night and among the remotest regions, as by his compass he can always ascertain the course his vessel is taking, and by altering the bearing of the helm, and shifting his sails, he keeps his ship constantly under command, and guides her to her destined haven.

THERE is another facility given to the mariner, which, if not so absolutely necessary to his progress as the compass, tends to relieve him from much of that danger to which he is continually exposed. This is the lighthouse erected along the sea-coast, or on some rock far away from the shore, over which the waves of the tempestuous ocean are unceasingly rolling, and which is placed there to warn passing or approaching vessels of shoals or other dangers that might cause their destruction. A few centuries ago, in and around England the sea and the land were alike dark. The bluff headlands of our coast looked over the sunken rocks, and the dangerous shoals -the shifting seasands had no friendly light to throw its golden streak upon the boiling eddies, or warn the traveller where Death was ever

waiting for his prey. The billows broke, booming upon the beach, over the wrecked vessel; for then, instead of life-boats manned with brave men, who from childhood have been familiar with the dangers of the deep, there were cruel wreckers prowling upon the shore in the darkness, ready to slay and rob the half-drowned mariners rather than to rescue them.

One of the earliest lighthouses of which we have any account was built on a rock called Pharos, opposite the city of Alexandria, about the year 283 B.C., in the time of Ptolemy Philadelphus, king of Egypt. This island was something short of a mile from the city, to which it was joined by a causeway, and upon the rock, of which it chiefly consists, Sostratus, the son of Dexiphanes, built a tower of white marble, which was considered one of the seven wonders of the world. It had several stories one above another, adorned with columns and balustrades, and galleries, formed of the whitest marble, wrought into the most beautiful workmanship. On the top, fires were kept constantly burning to direct sailors how to gain the harbour of Alexandria, which was at that time exceedingly difficult of access. And such was the splendour of the light, that it is said to have been visible at the distance of nearly a hundred miles, a fact that appears to be incredible. No pains were spared to render this tower as substantial and beautiful as possible, and the erection is calculated to have cost as much as eight hundred talents, which, if they are to be considered as Attic talents, were equal to £165,000 of our money, or if they were Egyptian coins, would amount in value to more than £300,000 sterling. Its fame indeed became so general, that its name was adopted as a generic term, and every lighthouse was afterwards known, almost till our own day, by the appellation of Pharos.

As the arts improved, so did the construction of these edifices progress, until one of the greatest accomplishments of engineering skill, ever attempted upon such works, was exhibited