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Long. in time, W. 5 10 = 1° 17' W. To find the sun's true azimuth.

O's alt,

15° 54' 18" sect. 016953 Lat.

48 58 25 sect. •182827 O polar dist. 82 4 18

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Variation

. 30 18 19 W., or nearly 24 points W. ON NAUTICAL INSTRUMENTS for Celestial and consequently by geometry the angle Is OBSERVATIONS.

half the angle E. The instruments used by seamen for celestial It evidently follows, from what has just been observations are, the quadrant, the sectant, and demonstrated, that the plane of a distant objeví the reflecting circle, which are all essentially the as seen by reflection from two parallel mirros same instrument, and depend on the following will be the same as that of the object itself

; a. general principles, viz. if an object be seen by consequently, if the image of a distant object a reflection from two mirrors, the angular distance seen by reflection from two mirrors coincide with of ihe object from its reflected image is double the object, we are certain that the mirrors 272 the inclination of the mirrors.

parallel to each other. For let A B, C D (fig. 18, plate II.), be two If C D' be a mirror perpendicular to C!! mirrors, whose planes produced meet in I. Let then an eye at S' would see the image of S in the S F be a ray of light from an object S, reflected direction of S'GE, in which case the supple from the mirror A B, in F G, to the mirror CD, ment of the distance of the object from its imar and again from C D, in D É, meeting S F pro- would be double the inclination of the mirrors

, duced in E. Then to an eye at E the angle and consequently a distant object and its image as SEG, or SES', would be the angular distance seen by reflection from two mirrors perpendică of S from its image S, as seen in the direction lar to each other would appear 180° apart, of EGS', after reflection from the two mirrors. diametrically opposite to each other. Now from the principles of optics, the angles Fig. 1, plate III., is a representation of the SFA and G F B are equal; and by geometry quadrant as it is commonly fitted

úp. SF A and B F E are equal; hence Fi bisects the graduated arc or limb of the instruments the angle G FE. Again F G being produced to A B a mirror perpendicular to the plane of the H, by the principles of optics, the angles FGC instrument, attached to the flat bar K, which nu and 'EGI are equal, and by geometry FGC and HG I are equal; 'hence # G E, the outward

volves with it round the centre, and carries at is angle of the triangle G F E, is bisected by GI, visions on the limb. E is a mirror also perpeto

POs

excess

dicular to the plane of the instrument, and having mirrors will be parallel to each other. Incline its lower part silvered, but its upper part transpa- the instrument to the horizon with the graduated rent; and it is parallel to A B when the zeros side of the limb upwards; and, looking as before of the vernier and limb coincide. G is another through the right vane and horizon glass, obmirror which is perpendicular to A B, when the serve whether the object and its image continue zeros of the vernier and limb coincide ; it has a still in the same straight line : if they do, the narrow transparent slit in the middle being the horizon glass is perpendicular to the plane of silvered both above and below the slit. H and the instrument. If the reflected image appears I are two sight vanes, which are sometimes fur- as in A c', fig. 5 plate III., the horizon glass indisbed with a moveable dark glass to admit of clines forwards; if as in A c it inclines backwards. the instrument being used in taking the sun's When it inclines forward slacken the screw n, altitude by an artificial horizon. A B is called fig. 1 plate III., before the horizon glass, and the index glass, E the fore horizon glass, and G tighten m behind it correspondently till A c corthe back horizon glass. I and l' are radii of responds with AC or B A produced. If the mirror the instrument, and M, N, braces or frames. R inclines backwards slacken m and tighten n til. is a small pencil to write down the observations the object and its image appear in a straight line, when taken. D is a series of dark glasses or in whatever way the instrument may be inclined. skreens which can be used singly or combined; Sometimes, instead of making the parallel adjustthey are interposed between A B and E; and ment of the horizon glass, the index is moved when the mirror G is used the skreens are in- forward or backward till the object and its image serted in a hole made to receive them at x. appear to coincide, when, the perpendicular ad

To make A B perpendicular to the plane of justment of the horizon glass being made, the the instrument, set the index forward towards distance of the zero of the index from the zero the middle of the limb as at Q; then, looking of the limb is called the index error of the inobliquely into A B, observe whether the image strument, to be added to all angles measured by of P Q, as seen by reflection in the mirror, is in and read from the instrument when the index the saine plane with P Q itself, as seen directly stands to the right, and subtracted when it stands by the eye. For example, in fig. 19, plate 11., to the left of the zero of the limb. A B represents the mirror, E the eye, and P'D The quadrant is commonly graduated to 20°, seen in the mirror the reflected image of PD and subdivided by means of the vernier to single seen by the eye; then, if PD and P D appear minutes. The graduations are generally conone continued plane, the mirror is perpendicular tinued a few degrees to the right of zero, and to the plane of the instrument, otherwise it is not; the prolongation of the arc is called the arc of if the reflected image appears the lower, the mirror is inclined backward ; if the higher it in- To adjust the back horizon glass G, or to clines forward. If it inclines backward, tighten make it perpendicular to A B, when the zeros the adjusting screw in the plane C by which the of the index and limb coincide, and also permirror is fastened to „ne index; but, if it inclines pendicular to the plane of the instrument. forward, slacken that screw till the image of the Place the zero of the index as much to the limb appears accurately in the same plane with right of zero on the limb as double the dip; then, the limb itself.

with an open horizon on both sides, look through Next make the zero of the index accurately the back sight vane I., through the slit in the coiucide with the zero of the limb, and the ob- middle of G, turn the glass by means of the ject then is to make the horizon glass E perpen- lever belonging to it at the limb of the instrudicular to the plane of the instrument and pa- ment till the horizon seen directly appears to rallel to the index glass A B. The horizon glass coincide with the opposite horizon as seen by E can be moved round by a lever attached to reflection, and, inclining the instrument, adjust an axis fastened to the frame in which E is set. the glass in the same manner as the fore horizon The axis passes through the frame of the instru- glass is adjusted till the horizon seen directly, ment, and the lever is attached to it on the other and the reflected image of the opposite horizon, apside. Unscrew the fastening screw of the lever, pear to coincide, when the glass will be adjusted. and looking through the sight vane H, and the In some instruments the adjusting levers are horizon glass E, at the horizon of the sea or any moved by means of a screw, and in the better other distant object, see if its image as seen by constructed instruments the small movements of the double reflection in the silvered part of the the index are also effected by a serew, called a horizon glass is in the same line with the object tangent screw attached at Q ; but, before this as seen directly through the unsilvered part. screw will act, the index must be clamped to the Let QO, fig. 4, plate III., represent the silvered limb, see fig. 2, plate III. when Q is the bead part of the horizon glass, R P its unsilvered part, of the tangent screw, and P that of the screw and B A the horizon seen through the unsilvered which clamps the index to the limb. part, and A C its image as seen by reflection in Fig. 2, plate III., is a representation of the the silvered part; then, if B A and AC are in sextant, in which the graduations are carried to a straight line, the horizon and index glasses are 120°. The essential parts of this instrument are parallel. But if the image of the object in the the same as those of the quadrant, and the mesilvered part of the glass appear above or below thods of adjusting its index and horizon glasses BA, as a' c, or ac, the index glass must be are effected in the same manner, though somemoved round by means of the lever at the back times by different and more delicate mechanical of the instrument till d'c' or ac correspond in processes. See LONGITUDE. F is a set of dark direction with AC or BA produced, when the glasses to be occasionally used before the horiVol. XV.

2 N

zon.

zon glass, in taking the index error of the in- down one or more of the dark glasses before strument by observations on the sun, and in the index mirror, the zeros of the index and taking the sun's altitude from an artifical hori- limb being brought together, look through the

Hl is a telescope screwed into a collar at right vane and horizon glass towards the sun, G, and this collar is attached to a stem x, which, and a colored image of it will be seen in the by means of a screw going perpendicularly silvered part of the reflector. love the index ihrough the plane of the instrument, can elevate forward till the colored image of the sun apor depress the telescope, and point it more or pears nearly in contact with the horizon. Then less towards the silvered or unsilvered part of vibrate the instrument a little on each side of the horizon glass, according as the object seen the vertical in a direction perpendicular to its directly, or that by reflection, may be required to own plane, and the image of the sun will appear be more or less bright: k is the eye-piece of the 10 describe a circular arch as A' A' A'' fig. 6, telescope which is to be drawn out iill distinct plate 11I. ; move the index till the lower edge vision is obtained. N is a microscope revolving just touches the horizon, when at the lowest part on a pin at M on the index, for more accurately of the arc, as at C; and the place of the index reading the graduations on the limb, and T is on the limb will show the altitude of the lower the handle by which the instrument is held. limb of the sun.

The telescope must be parallel to the plane of If the altitude of the upper limb is required, the instrument, and it is so placed by the follow- it may be taken in the same way, making the ing process.

In the focus of the eye-piece there image of the upper limb to coincide with the are four cross wires, so placed as to form a horizon, when at the lowest point of the are square on the centre of the field of view. Let which it appears to describe, as the instrument D E, fig. 20, plate 11., be the plane of the in- is vibrated from right to left. The sight vane strument, BB the telescope screwed into the generally has two holes, one at the same distance collar A A, x the stem of the collar passing by from the plane of the instrument as the upper means of a screw by which it can be raised or edge of the silvered part of the horizon glass, depressed through the plane of the instrument. the other opposite the middle of the unsilvered Turn the eye-piece of the telescope till two of part; and, as the eye and the image of the obthe wires in its focus, as a b, c d, appear parallel ject observed was presumed in the use of the to D Е, when the other two ct; gh, will of instrument to be at the same distance from its course be perpendicular to it. When the sun and plane, if the observer ook through the lower moon are at a considerable distance from each hole, the image ought to be kept on QR, fig. 4, other, bring the moon and the sun's image exactly plate III. if through the upper, midway between in contact on ab, and immediately bringing them M P and QR, through the whole arc of apparent to cd, if they still appear in contact, the telescope vibration A’A" A", fig. 6. is adjusted; if they appear to separate at cd, In taking the meridian altitude, the image of ighten the screw 1, and slackenm; if they over- the sun is brought, as above, in accurate contact lap at ed, slacken n and tighten 111, till the con- with the horizon, a little before the object attact appears perfect at both wires, when the tains its greatest altitude, and kept by succestelescope will be parallel to the plane of the sively and slowly advancing the index in contact instrument. Fig. 3, plate 111., is a representation as long as the altitude increases; after which, of Troughton's reflecting circle: C, C', C,'' are or as soon as the image appears to dip within three indexes attached to each other, and placed the horizon, the instrument is read for the meas nearly as they conveniently can be placed, at ridian altitude. equal distances on the arc C, carrying the tan- The sextant is used in the same manner, either gent screws (2, X, and the clamp P is used for when a plain open tube, or a direct telescope, reading the degrets, minutes, and seconds, by is applied in place of the astronomical or inthe others C and C", only the minutes and vertiny telescope, which is generally used with seconds are read ; C' is called the leading index, that instrument in observiny, A B is the index reflector, T the horizon glass, In taking the distance of any two celestial R and S the skreens as in the sextant. H is the objects, by means of the sextani, look through handle by which the instrument is held when the the telescope towards the dimmer object, and, instrument is in the position represented in the holding the plane of the instrument in the plane figure, Il” that by which it is held when in ob- passing through the two objects and the eye of serving the face is reversed, II" is the handle by the observer, move forward the index till the which it is held when the places of the indexes image of the other object appears nearly in conon the limb are read, the graduations being on tact with the object seen directly by the telethe opposite face of the limb. M', M", &c., are scope through the transparent part of the horithe microscopes for more accurately reading the zon glass. Then tighten the clamp screw of the subdivisions on the vernier. Il is the head of a index, P, fig. 2 plate 111., move the index slowly screw attached to the collar into which the tele- by means of the screw (, till the objects are in scope is screwed, and it is used for raising or

The object seen directly ought depressing the telescope to place it opposite that to be kept steadily in the centre of the field of part of the horizon glass that may best suit for view, and, by a slight motion of the wrist of that observing; II" I'" is a bent handle termi- hand by which the instrument is held, the image nating in II, having the bend sufficiently open of the object seen by reflection may be made io to admit the apparaius QPQ passing between pass and repass the other object, till in passing l'and I".

they are in exact contact, when the place of the To take the altitude of the sun by the quadrant. index on the limb will show the distance of the -Ilold the instrument vertically, and turning object.

accurate contact.

If one of the objects is the sun, and the other In taking altitudes with a sextant or circle, the moon, it will generally be necessary to put by an artificial horizon, the telescope is pointed down one of the dark glasses, D. And, with the to the image of the object as reflected from the moon and a star, it will often be necessary to surface of a Auid, or a polished plane set horiput down one of the lighter of the screens D, zontally by means of levels; and the image of to reduce the glare of the moon's light, that the object as reflected from the index glass to the the star may be distinctly enough seen when in horizon glass, and hence to the eye through the contact with the moon's image.

telescope, is brought in apparent contact with the The nearest limbs of the sun and moon are image of the object as seen by reflection from always brought in contact, and the enlightened the horizontal plane; and the distance of these or round limb of the moon is always brought in two images is double the altitude of the object. contact with a star, and, by applying the known In all cases it is recommended to use the insemi-diameters, the central distance of the ob- verting or astronomical telescope. The direct ject is obtained.

telescope is in general simply an opera glass, It is much more easy to observe with the face without means of ascertaining the position of its of the instrument upwards than downwards, line of collimation; and its field of view is ne

though this latter position, in the method of ob- cessarily small, as that of all telescopes con• serving detailed above (that which is almost structed on such principles must be. A very

universally practised), must necessarily be as- little practice will render the use of the invertsumed when the dimmer object is to the right. ing telescope easy. But the writer of this article in such circum- We have now only to describe the method of stances frequently puts one or more of the dark adjusting the back horizon glass of the quadrant glasses, F, before the horizon glass, and, removing G, fig. 1, plate III. To do this, place the zero those at D from before the index glass, looks of the index to the right of zero, on the limb, a directly at the brighter object, and takes the quantity equal to twice the dip; then looking dimmer one by reflection. In this case, how- through the sight vane I, and the slit in the ever, it is necessary to bring the telescope near middle of G at the horizon, move the glass G the plane of the instrument by means of the by means of the lever at the back of the instruscrew attached 10 X, the stem of the collar G, ment till the opposite horizon, or that behind that a considerable portion of the object glass the observer, appear in a line with that seen diof the telescope may be opposite the silvered rectly through the transparent slit. Incline the part of the horizon glass.

instrument to the horizon, and, if the horizon In taking distances with the reflecting circle, seen directly and by reflection do not then hold the instrument by the handle H, fig. 3, coincide, adjust by the screws before and behind plate III; and looking through the telescope and the glass, as in adjusting E, 'till they appear to the horizon glass T at the dimmer object, move coincide in all positions of the instrument; then forward the index Q PQ till the image of the the back-horizon glass G will be perpendicular brighter one, as seen by reflection from A B, the to A B when the index is at zero, and also perindex glass, appear in contact with the dimmer pendicular to the plane of the instrument. one seen directly, and, clamping the screw Z, To take an altitude by a back observation, make the contact perfect by means of either of look through the right vane F, fig. 1 plate III., the screws Q, Q. Then, taking the instrument and the transparent slit in the middle of the by the handle H", turn the graduated side of back horizon glass G, at that part of the horizon the instrument upwards, and read the degrees, opposite to the sun; and, moving the index KQ, minutes, &c., at the vernier attached to the index the image of the sun will appear to ascend ; make QPQ', but the minutes, &c., only at the vernier's the contact perfect in the same way as in the fore attached to C',C". Next take the instrument by observation. The apparent upper limb of the the handle H®, and, reversing it, point the tele- object is its real lower limb. scope again towards the dimmer object, and The distance of any two celestial objects may moving forward the leading index Q PQ, make be measured with a quadrant, though not with the contact perfect, and read the verniers as be- the same nicety as it may be done with a sexfore. Then the sixth part of the sum of the tant; if, however, the distance is more than 90°, readings will be the distance independent of it must with the quadrant be measured by a index error, which, when the instrument is used back observation, which is done by looking at in this manner, has no existence, as each of the the dimmer object, and moving the index till the indexes passes over twice the arch to be mea- reflected image of the other appears to coincide sured, the leading one to the right and left of with it; when the supplement of the angle zero, and the others in equal quantity on dif- shown by the index on the limb of the instruferent parts of the arch.

ment will be the required distance of the objects. The microscopes M', M", &c., are brought in NAVIGATION, INLAND. See Inland Navireading round to the verniers of the indexes to which they are attached.

NAVIGATION Law. See MARITIME LAW.

-1

GATION

Navigator's Islands, a cluster of ten losty an uncertain lat. southward. The easternmost of islands in the South Pacific Ocean; some of the cluster seem to have been discovered by Rogwhich are well-peopled and remarkable for their gewein and Bauman in 1722; another of magextent and fertility. They are situated between nitude was added by Bougainville in 1768, and 169° and 1720 30' W. long., and from 13° 25' to the two westernmost, which are the most consisiderable, were first seen by Perouse in 1787. to fifty guns: the sixth mate to include all skin Each of the last is more than forty miles in from twenty-four to thirty-six guns: and that the length. They all were visited by Edwards in complements of men be established as under: 1791. Perouse mentions three more of which he heard to the southward, named Sheka, Os

1st Rate, 900 - 850 or 800 men

2nd do. 700 or 650 samo, and Ooera. He speaks of the inhabitants

3d do. 650 or 600 of these islands as stout and well-made men,

4th do. 450 or 350 their ordinary height being five feet nine, ten, or

5th do. 300 or 280 eleven inches. Their bodies are painted or

6th do. tatooed : round their loins they wear a girdle of

175 — 145 or 125. sea-weed, which reaches to their knees. Their Of sloops the complements established accordin: hair is very long, and frequently turned up all to their size, to consist of 135, 125, 95, os in round the head, so as to heighten, say the sim- men, Brigs (not sloops), cutters, schooners, am ple navigators, the ferocity, or, as we might say, bombs, with 60 or 50 men. Thus stands that the dandyism, i. e. (so do extremes meet the rating and manning of the navy at present; bu Bond Street refinement of their countenances; another war, or a new administration of the which always express astonishment,' we are told, fairs of the navy, will, in all human probabin, or choler ! The least dispute between them is make new regulations. followed by violence: and often costs the com- 2. The personel of the navy, properly so calledbatants their lives; they are most universally, This consists of the commissioned Oficers; iz therefore, covered with scars, the consequences

the flag-officers, post-captains, commanders

, a. of these quarrels. The manners of both male lieutenants; the warrant-officers, petty oficers and female are disgustingly profligate. They dis

and seamen. dained the iron tools which were offered them Flag-officers are divided into those of the thre: in exchange for produce, and use hatchets shaped squadrons, red, white, and blue, each of fic like adzes, made of a very fine basaltes. The has three ranks of fag-officers; as admiral of the chief food is roots, hoys, and poultry: and they red, white, or blue; vice-admiral of the main manufacture a good sail-cloth for their canoes; white, or blue; rear-admiral of the red, wit which are also well-built and ornamented. or blue; the admiral wearing his color at t

Navy is used for the fleet or shipping of a main, the vice-admiral at the fore, and the reaprince or state. See Maring. The manage- admiral at the mizen-mast-head. There is ako ment of the British royal navy, under the lord an admiral of the feet, who, if in command

. high admiral of Great Britain, is often entrusted would carry the union Alag at the main. There to principal officers and commissioners, who hold are besides superannuated rear-admirals, enjortheir places by patent. The royal navy of Great ing the rank and pay of a rear-admiral

, bu s Britain is now in a very flourishing state, having capable of rising to a higher rank. There is also been diligently kept up in late reigns, as the na- in the navy the temporary rank of commodo, tural strength of the kingdom. When it is com- generally an old post-captain, and distinguished plete, it is divided into three squadrons, distin. by wearing a broad pendant.” He ranks nest w guished by the colors of the flags carried by the the junior rear-admiral, and above all posi-carespective admirals belonging to the same, viz. tains, except where the captain of the deet shali red, white, and blue; the principal commander be a post-captain, who, in that situation, takas of which bears the title of admiral : and each rank next to the junior rear-admiral. has under him a vice-admiral and a rear-admijal, The warrant officers are the master, second who are likewise Bag-officers.

master, gunner, boatswain, carpenter. There Navy. In our statistics of Great Britain are other warrant officers, who, though notwe have given an account of the rise and pro- combatants, constitute a part of the establisha gress of this important source of our national ment of the larger classes of ships of war. These strength and greatness. Our article Suip-Build- are, the chaplain, surgeon, surgeon's assistant Ing will enter fully into what the French call the purser. To which may be added, as part of the muteriel of this part of the service. We may staff of a fleet or squadron, secretary to the ad here notice

miral or commander-in-chief, and physician of 1. The present rating of the navy.--To the fleet. The petty officers are very numerous remedy the inconveniences resulting from the the principal of whom are master's mates and different scales formerly adopted in the mea- midshipmen. Their names or ratings will be surement of ships, the lords of the admiralty seen in the following table of the establishment suggested, by their memorial to the prince re- of the ratings and pay in the different classes of gent, the present rating, which, by his order in ships of war. council, of the 25th November 1816, was ordered The officers of the navy thus rank with those to be carried into effect, i. e. that the ships of the of the army:navy should for the future be raled as under:

Navy.

Army. The first rate to include all three-deckers, inas- Admiral of the fleet, Field-marshal. much as all sea going ships of that description Admiral,

General. carry 100 guns and upwards; the second rate to Vice-admiral,

Lieutenant-general

. include all ships of eighty guns and upwards, on Rear-admiral, Major-general. two decks; the third rate to include all ships of Commodore,

Brigadier-general

. seventy guns and upwards, and less than eighty Post-captain of 3 years, Colonel. guns: the fourth rate to include all ships of fifty Post-captain under do., Lieutenant-colonel. guns and upwards, but less than seventy guns : Commander,

Major. the fifth rate to include all ships from thirty-six Lieutenant,

Captain.

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