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Tutor. The mariner's compass consists of the box, the card or fly, and the needle. The box is circular, and is so suspended as to retain its horizontal position in all the motions of the ship. The glass is intended to prevent any motion of the card by the wind, the card or fly moves with the needle, which is very nicely balanced on a centre. It may, however, be noticed that a needle which is accurately balanced before it is magnetised, will lose its balance by being magnetised, on account of what is called the dippina Q
, therefore a small weight, or moveable piece of brass, is placed on one side of the needle, by the shifting of which the needle will always be balanced. CONVERSATION XXVI.
of the Variation of the Compass.
CHARLES. You said, I think, that the magnet pointed nearly north and south, how much does it differ from that line?
Tutor. It rarely points exactly north and south, and the deviation from that line is called the variation of the compass, which is said to be east or west.
James. Does this differ at different times ?
Tutor. It does; and the variation is very different in different parts of the world. The variation is not the same now that it was half a century ago, nor is it the same now at London that it is at Bengal or
Kamtschatka. The needle is continually traversing slowly towards the east and west.
This subject was first attended to by Mr. Burrows, about the year 1580, and he found the variation then, at London, about 11° 11' east. In year 1657, the needle pointed due north and south : since which the variation has been gradually increasing towards the west, and in the year 1803, it was equal to something more than 24° west, and was then advancing towards the same quarter.
Charles. That is at the rate of something more than ten minutes each year.
Tutor. It is, but the annual variation is not regular ; it is more one year than another. It is different in the several months, and even in the hours of the day.
James. Then if I want to set a globe due north and south, to point out the stars by, I must move it about, till the needle in the compass points to 24° west?
Tutor. Just so : and mariners, knowing this, are as well able to sail by the compass, as if it pointed due north.
Charles. You mentioned the property
which the needle had of dipping, after the magnetic fluid was communicated to it: is that always the same?
Tutor. It probably is, at the same place: it was discovered by Robert Norman, a compass-maker, in the year 1576, and he then found it to dip nearly 72, and from many observations made at the Royal Society, it is found to be the same.
James. Does it differ in different places?
Tutor. Yes. In the year 1773, obseryations were made on the subject, in a voyage toward the north pole, and from these it ap
In latitude 60° 18' the dip was 750.0
81 52 -89 27
I will show you an experiment on this subject. IIere is a magnetic bar, and a small dipping needle: if I carry the needle, suspended freely on a pivot, from one end of the magnetic bar to the other, it will, when directly over the south pole, settle
directly perpendicular to it, the north end being next to the south pole.
As the needle is moved, the dip grows less and less, and when it comes to the magnetic centre, it will be parallel to the bar; afterwards the south end of the needle will dip, and when it comes directly over the north pole, it will be again perpendicular to the bar.
The following facts are deserving of recollection.
1. Iron is the only body capable of being affected by magnetism.
2. Every magnet has two opposite points called poles.
3. A magnet freely suspended arranges itself so that these poles point nearly north and south. This is called the directive property, or polarity of the magnet.
4. When two magnets approach each other, the poles of the same names, that is, both north, or both south, repel each other.
5. Poles of different names attract each other.
6. The loadstone is an iron oar naturally possessing magnetism. VOL. III.