in astronomy, an arch of the horizon, intercepted between the meridian of the place and the azimuth, or vertical circle passing through the centre of the object, which is equal to the angle of the zenith, formed by the meridian and vertical circle: or it is found by this proportion, As the radius to the tangent of the latitude of the place, So is the tangent of the sun's or star's altitude, for instance, to the cosine of the azimuth from the south, at the time of the equinox.
Magnetical AZIMUTH, an arch of the horizon intercepted between the azimuth, or vertical circle, passing through the centre of any heavenly body, and the magnetic meridian. This is found by observing the object with an azimuth-compass.
AZIMUTH-Compass, an instrument for finding either the magnetical azimuth or amplitude of an heavenly object.
The learned Dr Knight invented some time since a very accurate and useful sea-compass, which is at present used in the navy, and will be described under the article COMPASS. This instrument, with the following contrivance added by the ingenious Mr Smeaton, answers the purposes of an azimuth and amplitude compass.
The cover of the wooden box being taken off, the compass is in a condition to be made use of in the binnacle, when the weather is moderate; but if the sea runs high, as the inner box is hung very free upon its centre (the better to answer its other purposes), it will be necessary to slacken the milled nut, placed upon one of the axes that support the ring, and to lighten the nut on the outside that corresponds to it. By this means, the inner box and ring will be lifted up from the edges, upon which they rest, when free; and the friction will be increased, and that to any degree necessary, to prevent the too great vibrations which otherwise would be occasioned by the motion of the ship.
To make the compass useful in taking the magnetic azimuth or amplitude of the sun and stars, as also the bearings of headlands, ships, and other objects at a distance, the brass edge, designed at first to support the card, and throw the weight thereof as near the circumference as possible, is itself divided into degrees and halves; which may be easily estimated into smaller parts, if necessary. The divisions are determined by means of a catgut line, stretched perpendicularly with the box, as near the brass edge as may be, that the parallax, arising from a different position of the observer, may be as little as possible.
Underneath the card are two small weights, sliding on two wires, placed at right angles to each other; which being moved nearer to, or farther from, the centre, counterbalance the dipping of the card in different latitudes, or restore the equilibrium of it, where it happens by any other means to be got too much out of level.
There is also added an index at the top of the inner box, which may be put on and taken off at pleasure; and serves for all altitudes of the object. It consists of a bar, equal in length to the diameter of the inner box, each end being furnished with a perpendicular slit, with a fit parallel to the sides thereof: one of the slits is narrow, to which the eye is applied; and the other is wider, with a small catgut stretched up the middle of it, and from thence continued horizontally from the top of one slit to the top of the other. There is also Azimuth. A line drawn along the upper surface of the bar. These four, viz. the narrow slit, the horizontal cat-gut thread, the perpendicular one, and the line on the bar, are in the same plane, which dips itself perpendicular to the horizon, when the inner box is at rest, and hangs free. This index does not move round, but is always placed on so as to answer the same side of the box.
When the sun's azimuth is desired, and his rays are strong enough to cast a shadow, turn about the wooden box till the shadow of the horizontal thread, or (if the sun be too low) till that of the perpendicular thread in one file, or the light through the slit on the other, falls upon the line in the index bar, or vibrates to an equal distance on each side of it, gently touching the box if it vibrates too far; observe, at the same time, the degree marked upon the brafs edge by the catgut line. In counting the degree for the azimuth, or any other angle that is reckoned from the meridian, make use of the outward circle of figures upon the brafs edge; and the situation of the index bar, with regard to the card and needle, will always direct upon what quarter of the compass the object is placed.
But if the sun does not shine out sufficiently strong, place the eye behind the narrow slit in one of the files, and turn the wooden box about till some part of the horizontal or perpendicular thread appears to intersect the centre of the sun, or vibrate to an equal distance on each side of it, using smoked glass next the eye if the sun's light is too strong. In this method, another observer will be generally necessary to note the degree cut by the nonius, at the same time that the first gives notice that the thread appears to split the object.
From what has been said, the other observations will be easily performed: only, in case of the sun's amplitude, take care to number the degree by the help of the inner circle of figures on the card, which are the complements of the outer to 90°; and, consequently, show the distance from east to west.
The azimuth of the stars may also be observed by night; a proper light serving equally for one observer to see the thread, and the other the degree upon the card.
It may not be amiss to remark farther, that, in case the inner box should lose its equilibrium, and consequently the index be out of the plane of a vertical circle, an accurate observation may still be made, provided the sun's shadow is distinct; for by observing first with one end of the index towards the sun and then the other, a mean of the two observations will be the truth.
Plate LXXVII. is a perspective view of the compass when in order for observation; the point of view being the centre of the card, and the distance of the eye two feet. A B is the wooden box. C and D are two milled nuts; by means whereof the axis of the inner box and ring are taken from their edges, on which they move, and the friction increased, when necessary. E F is the ring that supports the inner box. G H is the inner box; and I is one of its axes, by which it is suspended on the ring E F. The magnet or needle appears passing through the centre, together with a small brace of ivory, that confines the cap to its place.
The card is a single varnished paper, reaching as far as the outer circle of figures, which is a circle of thin brafs; the edge whereof is turned down at right angles to the plane of the card, to make it more stiff. O is a catgut line, drawn down the inside of the box, for determining the degree upon the brafs edge. P Q R S is the index bar, with its two files and catgut threads; which being taken off from the top of the box, is placed in two pieces, T and V, notched properly to receive it. W is a place cut out in the wood, serving as an handle.
Azimuth Circles, called also azimuths, or vertical 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 they are represented by the quadrant of altitude, when foreworn in the zenith. On these azimuths is reckoned the height of the stars and of the sun when not in the meridian.