of the ftar defcends, till you have brought it down to the horizon the index will then fhew the obferved altitude of the ftar. The corrections to be applied to the obferved altitude of the star are : the index error, the dip (thefe two give the apparent altitude); the refraction gives the true altitude; the fixed ftars have neither femidiameter nor parallax worth notice.

In taking the altitude of a ftar, or the moon, by night, always get as near the water as poffible; in moderate weather a grating may be flung over the fhip's fide, and an obferver fit upon it to take the altitudes; the fame may be done to take the altitude of the fun in an hazy horizon; for the nearer the eye is to the furface of the water, the nearer the true horizon will be to the eye.

Advice to Seamen in the Choice of their Quadrants and Sextants. The joints of the frame must be clofe, without the leaft opening or loofenefs, and the ivory on the arch and nonius inlaid and fixed, fo as not to rife at the ends, nor above the plane of the inftrument; all the divifions on the arch and nonius must be exceeding fine and ftraight, fo that when the index or nonius is set to any divifion on the arch, the divifions on the line that coincide may appear diftinct, for only the first and last line on the nonius will coincide with the other lines upon the arch, if the quadrant is well divided; likewife try in different parts of the arch, if the nonius, or index plate, cuts regularly in order with those on the arch: if they do not, the divifions are bad, and the quadrant ought to be rejected.

Again, look into the great fpeculum or index glafs flant-ways, holding it about ten or twelve inches from the eye, and obferve the image of fome diftant object; if the image appears clear and diftinct in every part of the glass, the fpeculum is good; but if it appears notched, or drawn with fmall lines, the glass is veiny, and must be rejected, if more images than one of the fame object are feen, it fhews that the two furfaces are not ground parallel; the other fpeculum may be examined in the fame manner.

Obferve the fun, or a candle, through the dark glasses severally, holding the glass about eight or ten inches from the eye; if they are veiny, the object will appear notched at the edges, but if clear and well defined, the glaffes are good.

Quadrants, like watches, may appear well to the eye, and yet be good for little; it is therefore much better to give two guineas and an half, or three guineas, for a good one, that will last a man for life, than purchase those wretched inftruments, made up at a low price, which cannot be depended on.

The furprizing improvements made in Navigation fince the year 1767, when the first Nautical Almanack was published by Dr. Mafkelyne, the prefent Aftronomer Royal, are beyond the moft fanguine expectations; and though feveral nations have contributed towards this important end, the English have (by the enCouragement held out by Parliament, and the great improvements

made in nautical inftruments and calculations) furpaffed them all; fo that by the help of the improved fextant, the Nautical Almanack, and the Tables contained in this book, a skilful and expert obferver can determine the longitude to a degree of accuracy that people unacquainted with the operation would fcarcely think poffible.

Hadley's fextant is conftructed on the fame principles as the quadrant; but as it is used to measure the angular distance between the fun and moon, or the moon and a star, in order to determine the longitude, the arch is extended to 120°, for the purpose of measuring their distance when greater than 90°; it is also provided with fome appendages not generally annexed to a quadrant, in order to take the obfervation with greater accuracy.

On the adjoining plate is reprefented a fextant, the frame of which is generally made of brafs; the arch BB is divided into 120°, each degree into three parts, of course equal to 20 minutes, which are again fubdivided by the nonius into every half minute, or 30 feconds; every fecond divifion, or minute, on the nonius, is cut longer than the intermediate ones; the nonius is numbered at every fifth of thefe longer divifions, from the right towards the left, with 5, 10, 15, and 20, the firft divifion towards the right hand being to be confidered as the index divifion.

This is the general way of graduating fextants; but for obtaining greater accuracy, fome are divided as follow: the arch contains 120°; each degree is fubdivided into 4, of courfe equal to 15', which are again fubdivided by the nonius into 15"; every fourth divifion or minute of the nonius, is longer than the intermediate ones; the nonius is numbered at every fifth of thefe long divifions, from the right towards the left, with 5, 10, 15; the firft divifion towards the right hand is to be confidered as the index divifion. The prefent mode of dividing the nonius of the fextant is thus: (beginning from the right hand towards the left) by taking fifteen divifions on the nonius, equal to fourteen on the arch, confequently one divifion on the arch will exceed one on the nonius by, that is, by of a minute, where the degrees on the arch are fubdivided into, equal to 15 minutes.

The nonius, till very lately, was divided as the quadrant.

In order to obferve with accuracy the contact of the limbs of any two objects, an adjufting-fcrew, L, is added to the index, by which it may be moved with greater regularity than it can by the hand; but this fcrew does not act until the index is fixed by the finger-fcrew M. Care thould be taken not to force the adjustingfcrew when it arrives at either extremity of its adjustment. When the index is to be moved any confiderable quantity, the ferew M, at the back of the fextant, muft be loofened; but when the index. is brought nearly to the divifion required, this back fcrew fhould be tightened, and the index moved gradually by the adjufting-fcrew. N. B. Many quadrants have an adjusting-fcrew.

In

In many fextants the lower part of the index glass, or that nearest the frame, is filvered as ufual, and the back furface of the upper part painted black; also a screen is fixed at the bafe of the index glafs, turning on its axis, and may be placed over the filver part when the fun's rays are ftrong, in which cafe the image is reflected from the polished furface of the upper part, and the error, which might probably arife from the planes of the glaffes not being parallel, is thereby avoided.

There are feveral coloured glasses at H, each of which is set in a different frame, turning on a centre; they are used to screen the eye from the brightness of the folar rays, and the glare of the moon, and may be used separately or together, as occafion requires.

There are other fuch glaffes placed behind the horizon glass at F, to weaken the rays of the fun or moon when they are viewed direly through the horizon glafs; the paler glafs is fometimes -ufed in obferving altitudes at fea, to take off the ftrong glare of the horizon.

The fextant is furnished with a plain tube, without any glaffes; and to render the objects ftill more diftinct, it has two telescopes, one representing the objects erect, or in their natural pofition, the other fhewing them inverted; it has a large field of view, and other advantages; a little ufe will foon accuftom the obferver to the inverted pofition, and the inftrument will be as readily managed by it as the plain tube alone. By a telefcope the contact of the images is more perfectly diftinguifhed; and by the place of the images in the field of the telescope it is easy to perceive whether the fextant is held in the proper plane for obferving. By fliding the tube that contains the eye-glaffes in the infide of the other tube, the object is fuited to different eyes, and made to appear perfectly diftinct and well defined.

The telescopes are to be fcrewed into a circular ring, at K; this ring refts on two points against an exterior ring, and is held thereto by two crews; by turning one and tightening the other, the axis of the telescope may be fet parallel to the plane of the fextant. The exterior ring is fixed on a brafs ftem that flides in a focket, and by means of the screw S, at the back of the fextant, it may be raised or lowered fo as to move the centre of the telescope to point to that part of the horizon glafs which fhall be judged the moft fit for obfervation.

A circular head, containing a plate, in which there are three coloured glaffes, and a fourth that is open, fometimes accompanies this fextant. This head is to be fcrewed on the eye-end of the tube, or on that of either telescope. The edge of the plate projects a little beyond the head on one fide, and is moveable by the finger, fo that the open ring, or any of the coloured glaffes, may be brought between the eye glaffes of the telescope and the

eye.

To thefe are added, a small screw-driver to adjust the screws,

and

and a magnifying glafs to read off the obfervation with greater

accuracy.

The Adjustments of a Sextant are to fet the index and horizonglaffes perpendicular to the plane of the inftrument, and their planes parallel to each other; by the fame method as the quadrant, only fcrewing on the plain tube or telescope; alfo to fet the axis of the telescope parallel to the plane of the inftrument; each of thefe parobfervation

ticulars must be examined before as taken, and the

adjustments, if requifite, be made.

Rec

For correcting the index error, fee the rules for adjufting Hadley's Quadrant.

To fet the Axis of the Telescope parallel to the Plane of the Sextant.

the

telescope, fhould be parallel to the plane of the inftrument, as a deviation in that refpest will occafion a confiderable error in the obfervation; and this is mot fenfible in large angles. To avoid which, an inverted telescope is ufed, in whofe field there are placed two wires parallel to each other, and equidiftant from the centre; to which are sometimes added two others, at right angles to thefe, but parallel to each other. By means of these wires the adjustment may be made thus: fcrew on the telescope, and turn the tube containing the eye glafs, till the wires are parallel to the plane of the inftrument; then take two objects, as the fun and moon, or the moon and a star, whofe angular diftance must not be less than 90°, because the error is more eafily difcovered when the distance is great; bring them exactly into contact on the wire which is nearest the plane of the inftrument, and fix the index; then, by altering a little the pofition of the fextant, bring them to appear on the wire farthest from the plane of the inftrument; if they remain still in contact, the axis of the telescope is parallel to the plane of the fextant; but if the limbs of the two objects appear to feparate at the further wire, it fhews that the object-end of the telescope inclines towards the plane of the fextant; this must be rectified by tightening the fcrew nearest the fextant, which is attached to the ring that holds the telescope, having previously flackened the fcrew farthest from it. If the images overtop each other when brought to the wire fartheft from the fextant, the object end of the telefcope is inclined from the plane of the fextant, and must be retified by Rackening the fcrew nearest the fextant, and tightening the other. Repeat this operation till the contact be rendered perfect on both wires, the axis of the telescope will then be truly ad justed.

In measuring angular diftances he of fight, or axis of the

To obferve the angular Distance between the Sun and Moon.

Screw on the inverted telescope, placing the wires parallel to the plane of the inftrument; then turn down the fcreens, according to the brightness of the fun; place the index at O on the arch, and if the fun's image be very bright, turn up the fcreen before the hori

201

zon glass, and with the fcrew S, raise the telescope to the transpa rent part of the horizon glafs. Having done this, hold the fextant fo that its plane may país through the two objects: if the fun be to the right hand of the moon, the fextant is to be held with its face upwards; but if it be to the left hand, the face is to be held downwards. With the inftrument in this pofition, look directly at the moon through the telescope, and move the index forward, till the fun's image is brought nearly in contact with the moon's neareft limb; then fix the index by the fcrew under the fextant, and make the contact perfect by means of the adjufting-fcrew; at the fame time move the fextant flowly, making the axis of the telefcope the centre of motion, by which means the objects will pass each other, and the contact be more accurately difcriminated. The index will fhew the observed distance of the fun and moon's nearest limbs, which you will read off with a magnifying glass.

Second Method.

It will perhaps be more eafy for those who are not accustomed to make obfervations of this kind, to find the diftance nearly, and fetting the index forward to it, to look directly towards the moon, holding the inftrument as before; the fun will then appear nearly in contact with it, and is to be made perfect by the method abovementioned. In the Nautical Ephemeris, the diftance of the fun and moon is fet down for every three hours of time at Greenwich, on fuch days as the moon is not more than 120°, nor less than 40° diftant from the fun, and may be found for any intermediate time by taking proportional parts; from thefe diftances you may compute roughly their diftance at the time of obfervation, thus: Turn the fhip's longitude into time by Tab. XVI. and add it to the time of obfervation, if the longitude be weft, but fubtract it if the longitude be caft, the fun or difference will give the time at Greenwich; then, by the Ephemeris, find the diftance nearly at that time, from which fubtract 30 minutes for the fun and moon's femi-diameters, and the remainder will give the distance of their nearest limbs at the time of obfervation.

If a number of obfervations are to be taken, the following method will not be found unacceptable: Having brought the objects into contact, as before directed, and noted down their apparent angular distance, advance or draw back your index two or three minutes, according as the objects are receding or approaching, and wait till they again come into contact, repeating the operation as often as judged neceflary, ufing the mean of all the obfervations to determine the longitude. This method will be found eafy and ac

curate.

NOTE.-The contact of the limbs must always be obferved in the middle, between the parallel wires.

To obferve the Distance between the Moon and a Star. Turn down the lighteft fcreen before the index glafs, and dire the tele cope to the ffar, holding the fextant in its proper pofition,

as