sometimes fixed to the side of a wall, and the plane of it erected exactly in the plane of the meridian, it in this case receives the name of mural quadrant. Tycho Brahe was the first person who applied this arch to a wall; and Flamsteed, the first in England, who fixed one up of 8 feet radius made by the late ingenious Mr. Graham, in the Royal Observatory at Greenwich. ASTRONOMICAL,or Equatorial, Sector,a very useful mathematical instrument, made by the late ingenious Mr. Graham, for finding the difference in right ascension and declination between two objects, the distance of which is too great to be observed by the micrometer. He constructed the Zenith Sector of 24 feet radius at the Royal Observatory at Greenwich, with which Dr. Bradley discovered the aberration of the fixed stars..

ASYLUM. See the Royal NAVAL ASYLUM at Greenwich.

ATAIR, or ALTAIR, the brightest star in the constellation Aquila, and of the first magnitude; and its meridian altitude is frequently observed by mariners, for ascertaining the latitude. Its right ascen sion in time, adapted to the beginning of the year 1812, is 295° 24′ 0′′,30; annual variation in right ascension, 43",78. Its declination, 8° 22′ 40" N.; annual variation in declination, +9,1.

ATHWART, (par le travers, Fr. from a and twert, Dan. transverse) across the line of the ship's course; as, we discovered a fleet standing athwart us," i.e. steering across our way.

66

ATHWART-Ilawse, the situation of a ship when she is driven by the wind, tide, or other accident, across the stem of another, whether they bear against, or are at a small distance from each other; the transverse position of the former with respect to the latter being principally understood.

ATHWART the fore foot, is generally applied to the fight of a cannon-ball, as fired from one ship across the line of another's course, but a-head of her, as a signal for the latter to bring to.

ATUWART-Ships, reaching across the ship, from one side to the other, or in that direction.

ATLAS, a large book containing maps and charts, of the principal coasts, harbours, &c. of the known world; as if they were viewed from the top of that celebrated mountain, which the ancients esteemed the highest in the world; or rather on account of their holding the whole world like Atlas..

ATMOSPHERE, (of aru, a vapour, and paiga, a sphere) that invisible elastic fluid which surrounds the earth, and encloses it on all sides.

It receives its name from the Greeks, in consequence of the vapours which are continually mixing with it. The ancients considered it as one of the four elements of which all things were composed; and some of them seem to have thought that it enters as a constituent principle into other bodies, or at least that air and other bodies are mutually convertible into

each other.

No experiments on its nature could well be made by the ancients, as they were unprovided with every instrument. fitted for such investigations, and unac quainted with the principles upon which their construction depended. But it has occupied a great deal of the attention of modern philosophers, and has given birth to some of the most brilliant discoveries that

grace the annals of science. Its weight was first ascertained by Galileo, and applied by Torricelli to explain the rise of water in pumps, and of mercury in barometrical tubes, and by Paschal to the mensu. ration of the height of mountains.

Its elasticity was accurately determined by Boyle. Halley and New ton explained the effects produced on it by moisture. Hooke, Newton, and Boyle, pointed out its relation to light, to sound, and to electricity. Its effect upon combustibles and animals was investigated by Hooke, Priestley, and Lavoisier. Its constituents were detected and measured by the experiments of Priestley, Lavoisier, and Cavendish. effect of heat on it was determined by Shuckburgh, Dalton, and Gay Lussac.

The

From the experiments of Sir George Shuckburgh Evelyn, (Phil. Trans. 1777 and 1798) made with a degree of precision and patient industry, which perhaps have never been surpassed, it appears, that at the temperature of 60°, when the barometer stands at 30 inches, the specific gravity of atmospherical air is 0 001208, that of water being 1.000, or its weight is to that of water as 1 to 828. Hence 100 cubic inches of it under that pressure and at that temperature weigh 30.5 grains: for a cubic inch of pure water at that temperature weighs 252 506 grains, according to experiments of Shuckburgh corrected by Mr. Fletcher. (NICHOLSON's Journal, IV. 35.)

The air when weighed, is supposed to be in its usual state of dryness; when very moist, its specific gravity is diminished. An exact knowledge of the weight of a given bulk of air is of great importance, because it enables us with much facility to ascertain the weight of all other aerial bodies: for it is easy to determine the relative weight of any elastic fluid to that of air..

If the atmosphere were of uniform density, it would be easy to ascertain, with the utmost accuracy, the height to which it extends; for the height of the atmosphere would obviously be to the height of the mercury in the barometer, as the specific gravity of common air is to the specific gravity of mercury. making the calculation on this supposition, it will be found that the height of the atmosphere is a little more than five miles. As the air, however, gradually diminishes in density, the atmosphere must reach to a much greater distance from the earth than five miles.

By

It appears from the duration of twilight, that. at the height of 44 miles, the atmosphere is sufficiently dense to intercept the light of the sun, and reflect it to the earth it should therefore appear, that it extends to a much greater height.

When a ray of light enters the atmosphere, it is bent from its course by the same cause which refracts the rays of light when they pass through any dense medium, such as glass or water. The refraction sustained by light at its first entrance into the atmos phere must be very small, from the extreme rarity of the air.. The deviation, however, will gradually increase as it penetrates the denser strata, and the ray will describe a path increasing in curvature as it approaches the earth. From this property of the atmosphere, the apparent altitude of the sun, moon, and stars, is greater than their real elevation, and they appear to be raised above the horizon when they are actually below it.

"If we consider all the causes," says La Place, "which disturb the equilibrium of the atmosphere; its greatest mobility arising from its fluidity and elasticity; the influence of heat and cold on its elasticity; the great mass of vapour that it alternately absorbs and deposes; and lastly, the changes which the rotation of the earth produces in the relative velocities of its particles, which for this reason are displaced in the direction of the meridians; we should not be surprised at the inconstancy and variety of its motions, which it would be very difficult to subject to any fixed and certain laws."

ATMOSPHERICAL CLOCK, the name of a machine proposed by Dr. Brewster, for measuring the mean temperature of the atmosphere during any given interval.

This machine records every variation of temperature that takes place during a given period, and the result indicated on the dial-plate is the exact average of all the heights of the mercury in the thermometer. The variations of heat and cold affect the pendulum, which may be either of the tabular or gridiron kind; and which is so constructed, as to render sensible, in the motion of the clock, the alternate contractions and dilatations which it undergoes. See the article CLOCK. ATRIP, (prepor. Fr. trippen, Dutch) is applied to the anchor and the sails: for the first, see the article The ANCHOR is a-trip. The topsails are said to be atrip, when they are just started from the cap. ATTACK, (attaque Fr.) an assault. Also the manner and disposition of a fleet or ship in action. To ATTACK, (attaquer, Fr.) to assault. Ex. We attacked the enemy's van.

To ATTEMPT. To attempt a passage. (tenter un passage, Fr.)

To ATTEND. To attend the signals. (Veiller aux signaux, Fr.)

AUGER, (augure, tarriére, Fr.) a wimble, car. penter's tool for boring.

AURIGA, the WAGGONER, in astronomy, the name of one of the constellations in the northern hemisphere, containing 26 stars, according to Tycho Brahe; 40, according to Hevelius; and 66 in the Britannic catalogue,

This is one of the 48 asterisms mentioned by all the ancient astronomers; and represented by the figure of an old man, in a kind of sitting posture, with a goat and her kids in his left hand, and a bridle in his right.

AURORA BOREALIS, an extraordinary luminous appearance or meteor, shewing itself in the night time, in northern latitudes, whence it has got its name of northern lights, or northern dawn. It is also known among the vulgar by the name of streamers, or merry-dancers.

The aurora borealis may, with propriety, be distinguished into two kinds, the tranquil and the varying. The tranquil shines with a mild steady light, very much resembling the clearness of twilight; and preserves, for a considerable time, the form in which it first appears, with little or no variation.

The varying aurora is still more remarkable in its appearance, and occasionally exhibits the most brilliant and rapidly diversified forms. The duration of the aurora is very various. Sometimes it is formed and disappears in the course of a few minutes. At

other times it lasts during the whole night, or even for two or three days together.

In high northern latitudes, as those of Sweden, Lapland, and Siberia, the aerora boreales are singu larly resplendent, and even terrific. They frequently occupy the whole of the heavens; and, according to the testimony of Maupertuis, Middleton, Krafft, and others, eclipse the splendour of the stars, planets, and moon, and sometimes even of the sun himself.

The most accountable of all the circumstances respecting the aurora borealis, is, that it is not much more than a century since this phenomenon has been observed to occur with any degree of frequency in our latitudes; for it cannot be supposed that so beautiful and striking a phenomenon would have passed unnoticed, and unrecorded, during the two preceding centuries, while men of science, and particularly astronomers, were so busily employed in examining every remarkable appearance of the heavens; or that the philosophers of Greece and Rome would have remained silent concerning so beautiful a meteor, had it been in any degree familiarly known to them.

It is in vain to account for their silence by saying, that they inhabited latitudes which are scarcely ever visited by this appearance; for the Romans not only visited, but long resided in, the north of Germany, and in Britain, where the aurora is now frequently seen in great splendour.

The following ingenious theory has been proposed, with a view to resolve this difficulty: There is a very remarkable analogy between the phenomena of electricity and those of magnetism, and, apparently, an intimate dependence of the one upon the other. There are two species of electricity, a positive and a negative; and two species of magnetic polarity, a north and a south. A body positively electrified repels another body positively electrified, and attracts one that is electrified negatively; while the north pole of one magnet repels the north pole, and attracts the south pole of another. The electric shock will deprive a magnetic needle of its power, or commu nicate it to it again, according to the direction in which it is laid; and during a thunder-storm, the magnetic needle is observed to be powerfully agi tated. Thus the intimate connexion between electricity and magnetism seems to be satisfactorily established. Again, that imaginary line, or circle, which traverses the earth irregularly from the north towards the south, and is called the line of no variation, because the magnetic needle, when placed upon it, points truly to the poles, is observed to have a gradual, and pretty regular revolution round the earth, performed in about 1000 years; so that, when 1000 years have elapsed, the line of no variation will have reached the same situation which it occupied at the beginning of that period.

This line seems to have a sort of controul over the coruscations of the aurora, which are observed to follow its direction in the heavens; and, as it were, to be attracted towards it, and regulated by its influence in their course. But it appears, by calculation, that during the scientific age of Rome, the only period at which accurate observations of the phenomena of the heavens could be made and recorded, the line of no variation runs across the continent of Europe; and, consequently, the aurora borealis, or stream of E

electric matter that passes from the north-pole towards the equator, would find a ready vent from the earth into the sky, through the mountains, and other pointed bodies which are so plentifully scattered over the land. By thus flowing constantly, and so greatly subdivided, it could not give rise to any striking atmospheric phenomena.

At present, the direction of this line is through the Atlantic ocean; and, of consequence, the electricity of the polar regions is not imperceptibly transmitted to the atmosphere, but proceeds in such masses, and at such irregular intervals, as to exhibit itself in the beautiful phenomena of the northern lights. A thousand years ago, the line of no variation, no doubt, occupied the same situation as it does at present; and the aurora then shone forth in all its splendour; but, at that period, the nations of Europe were sunk in ignorance and barbarism; and whatever phenomena the heavens presented were lost to posterity, from the rudeness and want of knowledge of the people of that age.

Thus, if this theory be true, when the line of no variation shall again pass over the land, the aurora borealis will become invisible for a time; and when this line reverts to its present situation over the ocean, the aurora will once more shine forth with its wonted lustre.

Maupertuis, Cavallo, Halley, Euler, Hawkesbee, Canton, Franklin, Kirwan, and Libes, have all endeavoured to account for this beautiful phenomena by a great variety of theories. Mr. Dalton has employed a new one in his Meteorological Observations and Essays, 1793; and another has lately been employed by M. Monge. See Leçons de Physique, par PUJOULZ, 1805.

AVAST, (tiens bon là, Fr.) the order to stop, or pause, in any exercise; as, a vast heaving! that is to say, desist, or stop from drawing in the cable or hawser, by means of the capstan, &c.

AVERAGE, in commerce, (avarie, Fr. averagium, Lat.) the accidents and misfortunes which happen to ships and their cargoes, from the time of their loading till their unlading. It is divided into three kinds. 1. The simple or particular average, which consists in the extraordinary expenses incurred for the ship alone, or for the merchandise alone; such as the loss of anchors, masts, and rigging, occasioned by the common accidents at sea; the damages which happen to merchandises by storms, capture, shipwreck, wet, or rotting; all which must be borne and paid by the thing that suffered the damage. 2. The large and common average, being those expenses incurred, and damages sustained, for the common good and security, both of the merchandise and vessel, consequently to be borne by the ship and cargo, and to be regulated upon the whole. Of this number are the goods or money given for the ransom of the ship and cargo; things thrown overboard for the safety of the ship; the expenses of unlading, or entering into a river or harbour, and the provisions and hire of the sailors when the ship is put under embargo. 3. The small averages, which are expences for towing and piloting the ship out of, or into harbours, creeks, or rivers; one-third of which must be charged to the ship, and two-thirds to the cargo. AVERAGE is more particularly used for a certain

contribution that merchants make proportionably towards their losses. It also signifies a small duty which those merchants, who send goods in another man's ship, pay to the master, for his care of them. over and above the freight. Hence it is expressed in the bills of lading, paying so much freight for the said goods, with primage and average accustomed.

A-WEATHER, (au vent, Fr.) signifies the situation of the helm when pushed to the weather side of the ship, in contradistinction to a-lee; as hard a-weather, &c.

AWEIGH, (l'ancre à quitté, Fr. of a and weigh) the state of the anchor when it is drawn out of the ground in a perpendicular direction, by the application of mechanical powers, as a capstan or windlass, to the cable within the ship; so that aweigh is synonimous to atrip.

AWNING, (tendelet, from aulne, Fr.) a canopy of canvas extending over the decks of a ship or boat in hot weather, for the convenience of the officers and crew, and to preserve the decks from being cracked or split, ebaroui, by the heat of the sun. The awning is supported by a range of light posts, called stanchions, which are erected along the ship's side on the right and left; it is also suspended in the middle by a complication of small cords, called a crow-foot.

That part of the poop-deck which is continued forward beyond the bulk-head of the cabin, is also called the awning. See the article CROWFOOT.

AXE, (grande hache ou coignée, Fr.) a well known carpenter's tool, both in respect to its shape and size. It is applied to the purpose of trimming off, and reducing to its proper size, every species of timber or plank.

AXIS, is either a real or supposed line passing through the centre of any cylindrical or globular body; in short, the centre of any body, when such centre can be determined so accurately, that the body, if required, may turn on the same.

AXIS of the World, in astronomy, is an imaginary right line, conceived to pass through the centre of the earth, and to terminate at each end in the surface. It always remains parallel to itself, and at right angles with the equator. About this line, as an axis, the earth performs its diurnal rotation, from west to east.

Axis of a Planet, is a right line drawn through its centre, about which the planet revolves. The Sun, Earth, Moon, Jupiter, Mars, and Venus, are known, by observation, to move about their several axis; and the like motion may be easily inferred of Mercury, Saturn, and the Georgium Sidus.

AY, ad. (perhaps from aio, Lat.) yes, and is always used in lieu thereof at sea, with a repetition, ay, ay, Sir.'

AZIMUTH, of the sun, or star, &c. is an Arabic word, used in astronomy, to denote an arch of the horizon, intercepted between the meridian of the place and the azimuth or vertical circle passing through the sun or star; and is equal to the angle at the zenith formed by the said meridian and vertical circle: or it is the complement to the eastern or western amplitude, and is thus found by trigonometry: As radius is to the tangent of the latitude, So is the tangent of the altitude of the sun or star, To the cosine of the azimuth from the south, at the time of the equinox:

And at all other times by the following rule in navigation:

Find the sum of the polar distance and altitude of the sun, and the latitude of the ship. Take the difference between half this sum and the polar distance. To the log. secant of the altitude add the log. secant of the latitude, the log. cosine of the half sum, and the log. cosine of the difference; then half the sum of these will be the log. sine of half the sun's true azimuth, to be reckoned from the south in north latitude; but from the north in south latitude, towards the east, in the morning, and towards the west in the afternoon. Then if the true and observed azimuths be both on the east, or both on the west side of the meridian, their difference is the variation; but if one be on the east, and the other on the west side of the meridian, their sum is the variation; and to know if it be east or west, suppose the observer looking towards that point of the compass representing the magnetic azimuth: then, if the true azimuth be to the right of the magnetic, the variation is east; but if the true be to the left of the magnetic, the variation is west.

AZIMUTH, magnetical, an arch of the horizon contained between the magnetical meridian, and the azimuth or vertical circle of the object; or its apparent distance from the north or south point of the compass. This is found by observing the sun, or star, &c. with an azimuth compass, when it is ten or fifteen degrees high, either before or after noon.

AZIMUTH Compass, (compas de variation, Fr.) an instrument adapted to find, in a more accurate manner than by the common sea-compass, the sun's or stars' magnetical amplitude, or azimuth.

It is also used to take the bearings of headlands, ships, and other objects at a distance.

An account of it was first published in the year 1736, by Edward Hanxley, of Wapping, in his book of navigation; but he does not say who was the inventor of it. Mr. Walker has greatly improved this instrument of late, so as to make it show the variation without any computation, as also the time from noon, and the sun's altitude. This improved meridional azimuth compass is now very generally used by the masters in the navy, and is composed of a common azimuth compass, on which is mounted an universal equatorial sun-dial, that finds its own meridian. The sun-dial is composed of three parts, viz. an equatorial semicircle, a semicircle of latitude, and a flat piece of brass, called a bridge.

The azimuth card is divided in the usual manner with points, quarter-points, and degrees; and also the brass circle round the card is divided into degrees, &c. It has a stop in the usual way to fix the card.

The equatorial semicircle of the dial part is divided from the ends to its centre, which is marked 90°, or 12 hours of time.

The latitude semicircle is also divided into twice 90°, beginning at its centre, both ways up to 90°.

The bridge is a flat piece of brass, fixed over the equatorial semicircle at right angles to it: it has a slit or opening along its middle from end to end; in this opening is fitted a small piece of brass or slider, which has a small glass lens fixed in its centre, whose focus is equal to the radius of the equatorial semicircle, and collects the sun's image to a point on the

divisions in observing. On the bridge is laid down the sun's declination on one side of it; and on the other side are the months and days answering to the declination, so that the middle of the slider may be set to either.

The parts of this instrument are adjusted to each other, and fixed by the maker, except balancing the compass, which must be examined at times; in order to which, set the 90° on the latitude semicircle to the Zero, and the slider on the bridge to the middle or ; set the bridge to face the sun nearly, turn the instrument round on its centre in the box, and note the degree cut by the sun's image, or light spot on the equatorial semicircle; then turn it half round, and see if the spot of light cuts the same division on the other part of the equatorial semicircle; if not, the compass is not truly balanced, and must be adjusted, so that the bright spot shall cut an equal division both ways.-This adjustment is made by means of a lead weight fixed under the bottom of the compass box, which may be moved at pleasure.

To find the Variation direct. The instrument being adjusted as before directed, set the slider to the sun's declination, or day of the month, (it matters not which, as one answers to the other) the latitude semicircle to the latitude of the place at the time, as near as you can by your log account; set the bridge to face the sun, and turn the instrument round on its centre, till the point of light from the lens rest on the graduated divisions on the equatorial semicircle, then the bridge, and zero's corresponding, will be due north and south, or in the true meridian; therefore, whatever degree the north point of the compass card stands to the right or left of the true north point on the box will be the variation east or west, and the time of day will be shown on the equatorial semicircle. This instrument may be used as an azimuth compass. When applied to this purpose, set the slider to the middle of the bridge, and the latitude semicircle to 90, and ship the sights on the bridge, and observe the sun's bearing, and stop the card in the usual way; then turn the instrument round till the point of light cuts the equatorial semicircle, and the degree it falls on will be the altitude of the sun's centre, which with the latitude and decli nation compute the true azimuth, and find the variation in the usual way.

The Magnetical Azimuth may be taken after the sun's altitude is observed, or any other person may read it off without stopping the card.

No great accuracy can be expected either in time or sun's altitude from this instrument, as the sun's image covers a degree or more on the equatorial semicircle: it is, however, near enough in general for the business of variation; but it is not suffi ciently accurate for the longitude, either by chronometers or lunar observations. See the article COMPASS.

AZIMUTH, or Verticle Circles, are great circles of the sphere intersecting each other in the zenith and nadir, and cutting the horizon at right angles. These azimuths are represented by the rhumbs on common sea-charts; and on the globe by the quadrant of altitude, when screwed in the zenith. On these azimuths is reckoned the height of the sun or stars, &c. when out of the meridian.

BAC

BACK ACK OF THE STERN-POST, (contre-étambord extérieur, Fr.) an additional piece behind the sternpost. The difficulty of procuring a stern-post of sufficient breadth in one piece has introduced this practice it is strongly bolted thereto, and the hinges, which support the rudder, are fixed to it, and it is tenanted into the keel. See the article STERN-POST. To BACK an anchor, (empenneller une ancre, Fr.) to carry out a small anchor, as the stream or kedge, ahead of the large one, by which the ship usually rides, in order to support it, and prevent it from loosening, or coming home, in bad ground. In this situation the latter is confined by the former, in the same manner that the ship is restrained by the latter. To BACK astern, in rowing, (scier à culer, Fr.) is to manage the oars in a direction contrary to the usual method, so as that the boat or vessel impressed by their force, shall retreat or move with her stern foremost.

BACK the starboard oars! (scie tribord! avec les avirons, Fr.) the command to confine the above management to the oars on the right hand side of the boat only, in order to turn her round more speedily to that direction.

To BACK and fill, (coiffer et faire servir les voiles, Fr.) is an operation generally performed in narrow rivers, when a ship has the tide in her favour, and the wind is against her-Exam. "We were obliged to back and fill occasionally to get up the Thames."

To BACK the sails, (mettre á scier, Fr.) is to arrauge them in a situation that will occasion the ship to retreat or move astern, in consequence of the tide or current in her favour, and the wind contrary, but, light. This operation is particularly necessary in narrow channels, when a ship is carried along sideways by the strength of that tide or current, and it becomes requisite to avoid any object that may intercept her course, as shoals, or vessels under sail, or at anchor: it is also necessary, in a naval engagement, to bring a ship back, so as to lie opposite to her adversary, when she is too far advanced in the line; and also in fleets under convoy, where ships are too much crowded, by the above operation they may be preserved from falling aboard each other. See the article ABACK.

BACK the main-topsail! (brasse le grand hunicr sur le mát! Fr.), the command to brace that sail in such a manner that the wind may exert its force against the fore-part of the sail, and by thus laying it aback materially retard the ship's course.

BACK-BOARD, (le dossier d'un bateau, Fr.) a piece of board of a semi-circular figure, placed

В А С

transversely in the after-part of a boat, like the back of a chair, and serving the passengers to recline against whilst sitting in the stern-sheets. See the article BOAT.

BACK-FRAME WHEEL, for laying cordage, from a six-thread ratline, to a two-inch rope, is about four or five feet in diameter, and is hung between two uprights, fixed by tenons, on a truck, and supported by a knee of wood. Over its top is a semicircular frame, called the head, to contain three whirls (that run on the brasses) with iron spindles, secured by a hasp and pin. They are worked by means of a leather band encircling the whirls and wheel. Three of the whirls are turned when hardening the strands, and only one when closing the rope, the strands being hung together on it. The truck, on which the backframe wheel is fixed, runs on four wheels, and is made of three-inch oak-plank, about nine feet long, and thirteen inches broad at one end, and eleven inches broad at the other.

BACKSTAFF, an instrument formerly used for taking the sun's altitude at sea; being so called because the back of the observer is turned towards the sun when he makes the observation. It was sometimes called Davis's quadrant, from its inventor, Captain John Davis, a Welchman, and a celebrated navigator, who produced it about the year 1590.

This instrument consists of two concentric arches of box wood, and three vanes; the arch of the longer radius is of 30 degrees, and the other of 60 degrees, making between them 90 degrees, or a quadrant. It is now seldom used, and is called the English quadrant by the French.

BACK-STAYS, (galhaubans, Fr.) from back and stay, long ropes extending from the topmastheads to the starboard and larboard sides of the ship, where they are farther extended to the channels; they are used to second the efforts of the shrouds, in supporting the masts, when strained by a weight of sail in a fresh wind.

They are usually distinguished into breast-backstays and after-back-stays: the intent of the first being to sustain the mast when the ship sails upon a wind; or, in other terms, when the wind acts upon the ship sideways; the second is to enable her to carry sail when the wind is further aft; and the third kind take that name from being shifted or changed from one side to the other, as occasion requires. See the article TRAVELLING BACK STAYS.

There are also back-stays for the top-gallant-masts, in large ships, which are fixed in the same manner with those of the top-masts.