DIPPING-NEEDLE, or Inclinator Needle, a magnetic needle, so hung, as that, instead of playing horizontally, and pointing out north and south, one end dips, or inclines to the horizon, and the other points to a certain degree of elevation above it.
The dipping-needle was invented in the year 1575 by one Robert Norman a compass-maker at Wapping. The occasion of the discovery, according to his own account, was, that it being his custom to finish and hang the needles of his compasses before he touched them, he always found, that immediately after the touch, the north-point would bend or incline downward, under the horizon; insomuch that, to balance the needle again, he was always forced to put a piece of wax on the south end as a counterpoise. The constancy of this effect led him at length to observe the precise quantity of the dip, or to measure the greatest angle which the needle would make with the horizon; and this at London he found to be 71° 50'. In 1723 Mr Graham made a great many observations on the dipping-needle, and found the angle to be between 71 and 75 degrees. Mr Nairne, in 1772, found it to be somewhat above 72°. It is not certain whether the dip varies, as well as the horizontal direction, in the same place. The trifling difference between Mr Norman and Mr Nairne would lead us to imagine that the dip was unalterable; but Mr Graham, who was a very accurate observer, makes the difference more considerable. It is certain, however, from a great number of experiments and observations, that the dip is variable in different latitudes, and that it increases in going northwards. It appears from a table of observations made with the marine dipping-needle in a voyage
Needle. voyage towards the north pole in 1733, that in lat. 60. 18. the dip was 75°; and in lat. 70. 45. it was 77° 52'; in lat. 80. 12. it was 81° 52'; and in lat. 80. 27. it was 82° 24'.
Several authors have endeavoured to apply this discovery of the dip to the finding of the latitude; and Mr Bond attempted to apply it to the finding of the longitude also; but for want of observations and experiments he could not make any progress. The affair was farther prosecuted by Mr Whifon, who published a treatise on the longitude, and for some time imagined it was possible to find it exactly by means of the dip of the needle; yet he at last despaired of it, for the following reasons. 1. The weakness of the magnetic power. 2. The conculsion of the ship, which he found it exceeding difficult to avoid so much as was necessary for the accuracy of the experiments. 3. The principal objection was an irregularity in the motions of all magnetic needles, both horizontal and dipping, by which they, within the compass of about a degree, vary uncertainly backward and forward; even sometimes, in a few hours time, without any evident cause. For a particular account of these variations both of the horizontal and dipping needle, see the article VARIATION.
Mr Nairne made a dipping-needle in 1772 for the board of longitude, which was used in the voyage towards the north-pole. This is represented Plate CCCXLV. fig. 2. The needle AA is 12 inches long, and its axis, the ends BB of which are made of gold alloyed with copper, rests on friction-wheels CCCC, of four inches diameter, each end on two friction wheels; which wheels are balanced with great care. The ends of the axes of the friction-wheels are likewise of gold alloyed with copper, and moved in small holes made in bell metal; and opposite to the ends of the axes of the needle and the friction-wheels, are flat agates, set in at DDD, finely polished. The magnetic needle vibrates within a circle of bell-metal, EEE, divided into degrees and half degrees; and a line, passing through the middle of the needle to the ends, points to the divisions. The needle of this instrument was balanced before it was made magnetic; but by means of a cross, the ends of which are EFFF, (contrived by the reverend Mr Mitchell) fixed on the axis of the needle, on the arms of which are cut very fine screws to receive small buttons, that may be screwed nearer or farther from the axis, the needle may be adjusted both ways to a great nicety, after being made magnetic, by reversing the poles, and changing the sides of the needle. GG are two levels, by which the line of 0 degrees of the instrument is set horizontal, by means of the four adjusting screws LLLL; H is the perpendicular axis, by which the instrument may be turned, that the divided face of the circle may front the east or west; to this axis is fixed an index I, which points to an opposite line on the horizontal plate K when the instrument is turned half round; MMMM are screws which hold the glass cover to keep the needle from being disturbed by the wind. When this needle is constructed for sea, it is suspended by an universal joint on a triangular stand, and adjusted vertically by a plumb-line and button above the divided circle and the dovetail work at the upper go; and the divisions on the circle are adjusted so as to be perpendicular to
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the horizon by the same plumb-line, and an adjoining screw; and when it is adjusted, a pointer annexed to a screw, which serves to move the divided circle, is fixed at the lowest go. Whenever the instrument is used to find the dip, it must be so placed that the needle may vibrate exactly in the magnetic meridian.