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dicular to the plane of the instrument, and having
its lower part silvered, but its upper part transpa-
rent; and it is parallel to A B when the zeros
of the vernier and limb coincide. G is another
mirror which is perpendicular to A B, when the
zeros of the vernier and limb coincide; it has a
narrow transparent slit in the middle being
'silvered both above and below the slit. H and
I are two sight vanes, which are sometimes fur-
nished with a moveable dark glass to admit of
the instrument being used in taking the sun's
altitude by an artificial horizon. A B is called
the index glass, E the fore horizon glass, and G
the back horizon glass. I and I' are radii of
the instrument, and M, N, braces or frames. R
is a small pencil to write down the observations
when taken. D is a series of dark glasses or
skreens which can be used singly or combined;
they are interposed between A B and E; and
when the mirror G is used the skreens are in-
serted in a hole made to receive them at r.

To make A B perpendicular to the plane of the instrument, set the index forward towards the middle of the limb as at Q; then, looking obliquely into A B, observe whether the image of PQ, as seen by reflection in the mirror, is in the same plane with P Q itself, as seen directly by the eye. For example, in fig. 19, plate II., A B represents the mirror, E the eye, and P'D seen in the mirror the reflected image of PD seen by the eye; then, if P' D and PD appear one continued plane, the mirror is perpendicular to the plane of the instrument, otherwise it is not; if the reflected image appears the lower, the mirror is inclined backward; if the higher it inclines forward. If it inclines backward, tighten the adjusting screw in the plane C by which the mirror is fastened to .ne index; but, if it inclines forward, slacken that screw till the image of the limb appears accurately in the same plane with the limb itself.

Next make the zero of the index accurately coincide with the zero of the limb, and the object then is to make the horizon glass E perpendicular to the plane of the instrument and parallel to the index glass A B. The horizon glass E can be moved round by a lever attached to an axis fastened to the frame in which E is set. The axis passes through the frame of the instrument, and the lever is attached to it on the other side. Unscrew the fastening screw of the lever, and looking through the sight vane H, and the horizon glass E, at the horizon of the sea or any other distant object, see if its image as seen by the double reflection in the silvered part of the horizon glass is in the same line with the object as seen directly through the unsilvered part. Let QO, fig. 4, plate III., represent the silvered part of the horizon glass, QP its unsilvered part, and BA the horizon seen through the unsilvered part, and A C its image as seen by reflection in the silvered part; then, if BA and AC are in a straight line, the horizon and index glasses are parallel. But if the image of the object in the silvered part of the glass appear above or below BA, as a' c', or ac, the index glass must be moved round by means of the lever at the back of the instrument till a'c' or ac correspond in direction with AC or BA produced, when the VOL. XV.

mirrors will be parallel to each other. Incline the instrument to the horizon with the graduated side of the limb upwards; and, looking as before through the right vane and horizon glass, observe whether the object and its image continue still in the same straight line: if they do, the the horizon glass is perpendicular to the plane of the instrument. If the reflected image appears as in A c', fig. 5 plate III., the horizon glass inclines forwards; if as in Ac it inclines backwards. When it inclines forward slacken the screw n, fig. 1 plate III., before the horizon glass, and tighten m behind it correspondently till A c' corresponds with AC or BA produced. If the mirror inclines backwards slacken m and tighten n til the object and its image appear in a straight line, in whatever way the instrument may be inclined. Sometimes, instead of making the parallel adjustment of the horizon glass, the index is moved forward or backward till the object and its image appear to coincide, when, the perpendicular adjustment of the horizon glass being made, the distance of the zero of the index from the zero of the limb is called the index error of the instrument, to be added to all angles measured by and read from the instrument when the index stands to the right, and subtracted when it stands to the left of the zero of the limb.

The quadrant is commonly graduated to 20', and subdivided by means of the vernier to single minutes. The graduations are generally continued a few degrees to the right of zero, and the prolongation of the arc is called the arc of excess

To adjust the back horizon glass G, or to make it perpendicular to A B, when the zeros of the index and limb coincide, and also perpendicular to the plane of the instrument.

Place the zero of the index as much to the right of zero on the limb as double the dip; then, with an open horizon on both sides, look through the back sight vane I., through the slit in the middle of G, turn the glass by means of the lever belonging to it at the limb of the instrument till the horizon seen directly appears to coincide with the opposite horizon as seen by reflection, and, inclining the instrument, adjust the glass in the same manner as the fore horizon glass is adjusted till the horizon seen directly, and the reflected image of the opposite horizon, appear to coincide, when the glass will be adjusted.

In some instruments the adjusting levers are moved by means of a screw, and in the better constructed instruments the small movements of the index are also effected by a serew, called a tangent screw attached at Q; but, before this screw will act, the index must be clamped to the limb, see fig. 2, plate III. when Q is the head of the tangent screw, and P that of the screw which clamps the index to the limb.

Fig. 2, plate III., is a representation of the sextant, in which the graduations are carried to 120°. The essential parts of this instrument are the same as those of the quadrant, and the methods of adjusting its index and horizon glasses are effected in the same manner, though sometimes by different and more delicate mechanical processes. See LONGITUDE. F is a set of dark glasses to be occasionally used before the hori

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zon glass, in taking the index error of the instrument by observations on the sun, and in taking the sun's altitude from an artifical horizon. III is a telescope screwed into a collar at G, and this collar is attached to a stem a, which, by means of a screw going perpendicularly through the plane of the instrument, can elevate or depress the telescope, and point it more or less towards the silvered or unsilvered part of the horizon glass, according as the object seen directly, or that by reflection, may be required to be more or less bright: k is the eye-piece of the telescope which is to be drawn out till distinct vision is obtained. N is a microscope revolving on a pin at M on the index, for more accurately reading the graduations on the limb, and T is the handle by which the instrument is held. The telescope must be parallel to the plane of the instrument, and it is so placed by the following process. In the focus of the eye-piece there are four cross wires, so placed as to form a square on the centre of the field of view. Let DE, fig. 20, plate II., be the plane of the instrument, BB the telescope screwed into the collar A A, the stem of the collar passing by means of a screw by which it can be raised or depressed through the plane of the instrument. Turn the eye-piece of the telescope till two of the wires in its focus, as a b, c d, appear parallel to DE, when the other two ef, gh, will of course be perpendicular to it. When the sun and moon are at a considerable distance from each other, bring the moon and the sun's image exactly in contact on ab, and immediately bringing them to cd, if they still appear in contact, the telescope is adjusted; if they appear to separate at cd, tighten the screw n, and slacken m; if they overlap at ed, slacken n and tighten m, till the contact appears perfect at both wires, when the telescope will be parallel to the plane of the instrument. Fig. 3, plate III., is a representation of Troughton's reflecting circle: C, C', C," are three indexes attached to each other, and placed as nearly as they conveniently can be placed, at equal distances on the arc C, carrying the tangent screws Q, Q', and the clamp P is used for reading the degrees, minutes, and seconds, by the others C and C", only the minutes and seconds are read; C' is called the leading index, A B is the index reflector, T the horizon glass, R and S the skreens as in the sextant. H is the handle by which the instrument is held when the instrument is in the position represented in the figure, H that by which it is held when in observing the face is reversed, H" is the handle by which it is held when the places of the indexes on the limb are read, the graduations being on the opposite face of the limb. M', M", &c., are the microscopes for more accurately reading the subdivisions on the vernier. H is the head of a screw attached to the collar into which the telescope is screwed, and it is used for raising or depressing the telescope to place it opposite that part of the horizon glass that may best suit for observing; I' I" I" is a bent handle terminating in H, having the bend sufficiently open to admit the apparatus QPQ' passing between

I' and I".

To take the altitude of the sun by the quadrant. -Hold the instrument vertically, and turning

down one or more of the dark glasses before the index mirror, the zeros of the index and limb being brought together, look through the right vane and horizon glass towards the sun, and a colored image of it will be seen in the silvered part of the reflector. Move the index forward till the colored image of the sun appears nearly in contact with the horizon. Then vibrate the instrument a little on each side of the vertical in a direction perpendicular to its own plane, and the image of the sun will appear to describe a circular arch as A' A” A”” fig. 6, plate III.; move the index till the lower edge just touches the horizon, when at the lowest part of the arc, as at C; and the place of the index on the limb will show the altitude of the lower limb of the sun.

If the altitude of the upper limb is required, it may be taken in the same way, making the image of the upper limb to coincide with the horizon, when at the lowest point of the arc which it appears to describe, as the instrument is vibrated from right to left. The sight vane generally has two holes, one at the same distance from the plane of the instrument as the upper edge of the silvered part of the horizon glass, the other opposite the middle of the unsilvered part; and, as the eye and the image of the object observed was presumed in the use of the instrument to be at the same distance from its plane, if the observer look through the lower hole, the image ought to be kept on QR, fig. 4, plate III. if through the upper, midway between MP and Q R, through the whole arc of apparent vibration A'A" A", fig. 6.

In taking the meridian altitude, the image of the sun is brought, as above, in accurate contact with the horizon, a little before the object attains its greatest altitude, and kept by succes sively and slowly advancing the index in contact as long as the altitude increases; after which, or as soon as the image appears to dip within the horizon, the instrument is read for the me ridian altitude.

The sextant is used in the same manner, either when a plain open tube, or a direct telescope, is applied in place of the astronomical or inverting telescope, which is generally used with that instrument in observing.

In taking the distance of any two celestial objects, by means of the sextant, look through the telescope towards the dimmer object, and, holding the plane of the instrument in the plane passing through the two objects and the eye of the observer, move forward the index till the image of the other object appears nearly in contact with the object seen directly by the telescope through the transparent part of the hori zon glass. Then tighten the clamp screw of the index, P, fig. 2 plate III., move the index slowly by means of the screw Q, till the objects are in accurate contact. The object seen directly ought to be kept steadily in the centre of the field of view, and, by a slight motion of the wrist of that hand by which the instrument is held, the image of the object seen by reflection may be made to pass and repass the other object, till in passing they are in exact contact, when the place of the index on the limb will show the distance of the object.

If one of the objects is the sun, and the other. the moon, it will generally be necessary to put down one of the dark glasses, D. And, with the moon and a star, it will often be necessary to put down one of the lighter of the screens D, to reduce the glare of the moon's light, that the star may be distinctly enough seen when in contact with the moon's image.

The nearest limbs of the sun and moon are always brought in contact, and the enlightened or round limb of the moon is always brought in contact with a star, and, by applying the known semi-diameters, the central distance of the object is obtained.

It is much more easy to observe with the face of the instrument upwards than downwards, though this latter position, in the method of observing detailed above (that, which is almost universally practised), must necessarily be assumed when the dimmer object is to the right. But the writer of this article in such circumstances frequently puts one or more of the dark glasses, F, before the horizon glass, and, removing those at D from before the index glass, looks directly at the brighter object, and takes the dimmer one by reflection. In this case, however, it is necessary to bring the telescope near the plane of the instrument by means of the screw attached to X, the stem of the collar G, that a considerable portion of the object glass of the telescope may be opposite the silvered part of the horizon glass.

In taking distances with the reflecting circle, hold the instrument by the handle H, fig. 3, plate III; and looking through the telescope and the horizon glass T at the dimmer object, move forward the index Q P Qʻ till the image of the brighter one, as seen by reflection from A B, the index glass, appear in contact with the dimmer one seen directly, and, clamping the screw Z, make the contact perfect by means of either of the screws Q, Q. Then, taking the instrument by the handle I", turn the graduated side of the instrument upwards, and read the degrees, minutes, &c., at the vernier attached to the index QPQ', but the minutes, &c., only at the vernier's attached to C', C". Next take the instrument by the handle H", and, reversing it, point the telescope again towards the dimmer object, and moving forward the leading index QPQ', make the contact perfect, and read the verniers as before. Then the sixth part of the sum of the readings will be the distance independent of index error, which, when the instrument is used in this manner, has no existence, as each of the indexes passes over twice the arch to be measured, the leading one to the right and left of zero, and the others in equal quantity on different parts of the arch.

The microscopes M', M", &c., are brought in reading round to the verniers of the indexes to which they are attached.

NAVIGATOR'S ISLANDS, a cluster of ten lofty islands in the South Pacific Ocean; some of which are well-peopled and remarkable for their extent and fertility. They are situated between 169° and 172° 30′ W. long., and from 13° 25' to

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by an artificial horizon, the telescope is pointed In taking altitudes with a sextant or circle, to the image of the object as reflected from the surface of a fluid, or a polished plane set horizontally by means of levels; and the image of the object as reflected from the index glass to the horizon glass, and hence to the eye through the image of the object as seen by reflection from telescope, is brought in apparent contact with the the horizontal plane; and the distance of these two images is double the altitude of the object.

verting or astronomical telescope. The direct In all cases it is recommended to use the inwithout means of ascertaining the position of its telescope is in general simply an opera glass, line of collimation; and its field of view is necessarily small, as that of all telescopes constructed on such principles must be. A very little practice will render the use of the inverting telescope easy.

adjusting the back horizon glass of the quadrant We have now only to describe the method of G, fig. 1, plate III. To do this, place the zero of the index to the right of zero, on the limb, a quantity equal to twice the dip; then looking through the sight vane I, and the slit in the middle of G at the horizon, move the glass G by means of the lever at the back of the instrument till the opposite horizon, or that behind the observer, appear in a line with that seen directly through the transparent slit. Incline the seen directly and by reflection do not then instrument to the horizon, and, if the horizon coincide, adjust by the screws before and behind the glass, as in adjusting E, till they appear to coincide in all positions of the instrument; then the back-horizon glass G will be perpendicular to A B when the index is at zero, and also pendicular to the plane of the instrument.

per

look through the right vane F, fig. 1 plate III., To take an altitude by a back observation, and the transparent slit in the middle of the back horizon glass G, at that part of the horizon opposite to the sun; and, moving the index K Q, the image of the sun will appear to ascend; make the contact perfect in the same way as in the fore observation. The apparent upper limb of the object is its real lower limb.

be measured with a quadrant, though not with The distance of any two celestial objects may the same nicety as it may be done with a sextant; if, however, the distance is more than 90°, it must with the quadrant be measured by a back observation, which is done by looking at the dimmer object, and moving the index till the reflected image of the other appears to coincide with it; when the supplement of the angle shown by the index on the limb of the instrument will be the required distance of the objects. NAVIGATION, INLAND. See INLAND NAVI

GATION.

NAVIGATION LAW. See MARITIME LAW.

an uncertain lat. southward. The easternmost of the cluster seem to have been discovered by Roggewein and Bauman in 1722; another of magnitude was added by Bougainville in 1768, and the two westernmost, which are the most consi2N2

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