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by the wind, the card or fly moves with the needle, which is very nicely balanced on a centre.
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 dipping, 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.
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. Burrowes, about the year 1580, and he found the variation then, at London, about 11° 11' east. In the 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 va. riation 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 bad 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
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, observations were made on the subject, in a voyage toward the north pole, and from these it appears that
In latitude 60° 18' the dip was 75° 0'
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