NEEDLE

, Magnetical, denotes a Needle, or a slender piece of iron or steel, touched with a loadstone; which, when sustained on a pivot or centre, upon which it plays round at liberty, it settles at length in a certain direction, either duly, or nearly north-andsouth, and called the magnetic meridian.

Magnetical Needles are of two kinds; Horizontal and Inclinatory.

Horizontal Needles, are those equally balanced on each side of the pivot which sustains them; and which, playing horizontally, with their two extremes point out the north and south parts of the horizon.

Construction of a Horizontal Needle. Having procured a thin light piece of pure steel, about 6 inches long, a perforation is made in the middle, over which a brass cap is soldered on, having its inner cavity conical, so as to play freely on the style or pivot, which has a fine steel point. To give the Needle its verticity, or directive faculty, it is rubbed or stroked leisurely on each pole of a magnet, from the south pole towards the north; first beginning with the northern end, and going back at each repeated stroke towards the south; being careful not to give a stroke in a contrary direction, which would take away the power again. Also the hand should not return directly back again the same way it came, but should return in a kind of oval figure, carrying the hand about 6 or 8 inches beyond the point where the touch ended, but not beyond on the side where the touch begins.

Before touching, the north end of the Needle, in our hemisphere, is made a little lighter than the other end; because the touch always destroys an exact balance, rendering the north end heavier than the south, and thus causing the Needle to dip. And if, after touching, the Needle be out of its equilibrium, something must be filed off from the heavier side, till it be found to balance evenly.

Needles may also acquire the magnetic virtue by means of artificial magnetic bars in the following manner: Lay two equal Needles parallel and about an inch asunder, with the north end of one and the south end of the other pointing the same way, and apply two conductors in contact with their ends: then, with two magnetic hard bars, one in each hand, and held as nearly horizontal as can be, with the upper ends, of contrary names, turned outwards to the right and left, let a Needle be stroked or rubbed from the middle to both ends at the same time, for ten or twelve times, the north end of a bar going over the south end of a Needle, and the south end of a bar going over the north end of a Needle: then, without moving from the place, change hands with the bars, or in the same hands turn the other ends downwards, and stroke the other Needle in like manner; so will they both be magnetical. But to make them still stronger, repeat the operation three or four times from Needle to Needle, and| at last turn the lower side of each Needle upwards, and repeat the operations of stroking them, as on the former sides.

The Needles that were formerly applied to the compass, on board merchant ships, were formed of two pieces of steel wire, each being bent in the middle, so as to form an obtuse angle, while their ends, being applied together, made an acute one, so that the whole represented the form of a lozenge. Dr. Knight, who has so much improved the compass, found, by repeated experiments, that partly from the foregoing structure, and partly from the unequal hardening of the ends, these Needles not only varied from the true direction, but from one another, and from themselves.

Also the Needles formerly used on board the men of war, and some of the larger trading ships, were made of one piece of steel, of a spring temper, and broad towards the ends, but tapering towards the middle. Every Needle of this form is found to have fix poles instead of two, one at each end, two where it becomes tapering, and two at the hole in the middle.

To remedy these errors and inconveniences, the Needle which Dr. Knight contrived for his compass, is a slender parallelopipedon, being quite straight and square at the ends, and so has only two poles, although the curves are a little confused about the hole in the middle; though it is, upon the whole, the simplest and best.

Mr. Michell suggests, that it would be useful to increase the weight and length of magnetic Needles, which would render them both more accurate and permanent; also to cover them with a coat of linseed oil, or varnish, to preserve them from any rust.

A Needle on occasion may be prepared without touching it on a loadstone: for a fine steel sewing Needle, gently laid on the water, or delicately suspended in the air, will take the north-and-south direction.— Thus also a Needle heated in the fire, and cooled again in the direction of the meridian, or only in an erect position, acquires the same faculty.

Declination or Variation of the Needle, is the deviation of the horizontal Needle from the meridian; or the angle it makes with the meridian, when freely suspended in an horizontal plane.

A Needle is always changing the line of its direction, traversing slowly to certain limits towards the east and west sides of the meridian. It was at first thought that the magnetic Needle pointed due north; but it was observed by Cabot and Columbus that it had a deviation from the north, though they did not suspect that this deviation had itself a variation, and was continually changing. This change in the Variation was first found out, according to Bond, by Mr. John Mair, secondly by Mr. Gunter, and thirdly by Mr. Gellibrand, by comparing together the observations made at different times near the same place by Mr. Burrowes, Mr. Gunter, and himself, and he published a Discourfe upon it in 1635. Soon after this, Mr. Bond ventured to deliver the rate at which the Variation changes for several years; by which he foretold that at London in 1657 there would be no Variation of the compass, and from that time it would gradually increase the other way, or towards the west, making certain revolutions; which happened ac- cordingly: and upon this Variation he proposed a method of finding the longitude, which has been farther improved by many others since his time, though with very little success. See Variation.

The period or revolution of the Variation, Henry Philips made only 370 years, but according to Henry Bond it is 600 years, and their yearly motion 36 minutes. The first good observations of the Variation were by Burrowes, about the year 1580, when the Variation at London was 11° 15′ east; and since that time the Needle has been moving to the westward at that place; also by the observations of different persons, it has been found to point, at different times, as below:

Years.Observers.Variat. E. or W.
1580Burrowes11°15′ East.
1622Gunter556
1634Gellibrand4 3
1640Bond3 <*>
1657Bond0 0
1665Bond123 West.
1666Bond136
1672-230
1683-430
1692-600
1723Graham1417
1747-1740
1774Royal Society2116
1775Royal Society2143
1776Royal Society2147
1777Royal Society2212
1778Royal Society2220
1779Royal Society2228
1780Royal Society2241

By this Table it appears that, from the first observations in 1580 till 1657, the change in the Variation was 11° 15′ in 77 years, which is at the rate nearly of 9′ a year; and from 1657 till 1780, or the space of 123 years, it changed 22° 41′, which is at the rate of 11′ a year nearly; which it may be presumed is very near the truth.

The Variation and Dip of the Needle was for many years carefully observed by the Royal Society while they met at Crane Court; and it is a pity that such observations have not been continued since that time.

Dipping, or Inclinatory Needle, is a Needle to shew the Dip of the Magnetic Needle, or how far it points below the horizon.

The Inclination or Dip of the Needle was first ob. served by Robert Norman, a compass-maker at Ra<*>cliffe; and according to him, the dip at that place, in the year 1576, was 71° 50′; and at the Royal Society it was observed for some years lately as follows:

viz in177672° 30′
177872 25
178072 17.

Mr. Henry Bond makes the Variation and Dip of the Needle depend on the same motion of the magnetic poles in their revolution, and upon it he founded a method of discovering the longitude at sea.|

NEEP Ti<*>des. See Neap Tides.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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NAVIGATION
NAVIGATOR
NEAP
NEBULOUS
NEEDHAM (John Tuberville)
* NEEDLE
NEGATIVE
NEWEL
NEWTON (Dr. John)
NEWTON (Sir Isaac)
NICHE