subtracted from all observations taken with the instrument, until it be found, by a similar process, that the index error has altered." "To make the Line of Collimation of the Telescope parallel to the Plane of the Sextant. This is known to be correct, when the sun and moon, having a distance of 90° or more, are brought into contact just at the wire of the telescope which is nearest the plane of the sextant (see diagram page 267), fixing the index, and altering the position of the instrument to make the objects appear on the other wire; if the contact still remains perfect, the axis of the telescope is in proper adjustment; if not, it must be altered by moving the two screws which fasten, to the up-and-down piece, the collar into which the telescope screws. adjustment is not very liable to be deranged." This OF THE USE OF THE SEXTANT. The large sextant is rarely required for observations (on land) of terrestrial or of celestial bodies; but, for purposes of navigation or for maritime surveying it is of essential importance. Instruments, in fact, constructed on this principle are the only instruments capable of being used on ship-board for determining altitudes, or measuring the angular distances of objects. Mode of Holding the Instrument. The instrument is held lightly in the right hand, and moved until its face is in the plane passing through the eye and the two objects of which the angular distance is required. When altitudes, therefore, are to be observed, the instrument is held in a vertical plane; when horizontal or oblique angles are to be measured, it is held in a horizontal or oblique plane. Mode of Observing with the Sextant. When the altitude of an object, (the sun, for instance,) is to be observed at sea, where no level or artificial horizon can be used, the observer having turned down one or more of the dark glasses, according to the brilliancy of the object, holds the instrument in a vertical plane passing through the sun, having the sea horizon before him. Directing his sight to that part of the horizon immediately beneath the sun, he then with the left hand lightly slides the index forward, until the image of the sun, reflected from the index-glass, appears in contact with the horizon, seen through the unsilvered part of the horizon-glass. He then clamps the index firm, and turns the tangent-screw carefully and lightly, to make the contact of the upper or lower limb of the sun and the horizon perfect, when it will appear a tangent to the circular disc. The angle read off, corrected from the index error, if any, is the observed altitude. If, instead, the angle between two terrestrial objects be required, the observer holds the instrument steadily in a plane passing through the objects and the eye, and directing his sight to the object on his left, he slides, with the left hand, the index forward until the two objects are brought nearly to coincide. He then clamps the index as before, and by the tangent screw renders the contact perfect. When the angular distance is thus observed, between terrestrial objects, for surveying or similar purposes, the only correction required when the objects are at the same level is that for the index error if any. When the objects are at different elevations, it may be requisite further to apply the reduction to the horizontal angle as explained in page 113. When the altitude of the sun however has been observed, there are certain other corrections required to obtain the true altitude before the observation can be made available for calculation or other purposes. These corrections are as follows: To or from the angle read add or subtract the sun's semidiameter, as given in the Nautical Almanac*, according as the lower or upper limb is observed, to obtain the apparent altitude of the sun's centre. Before we can use this observation for determining the time, the latitude, &c., it must be further corrected for refraction and for parallax caused by the distance of the observer from the earth's centre, to obtain the true altitude, subtracting the former and adding the latter; and when the sea horizon is employed, a quantity must also be subtracted for the dip, which is unnecessary when the altitude is taken by means of an artificial horizon. The correction for parallax is always additive. Let E be the earth's centre, A the place of observation on the earth's surface, S the sun, A B the artificial horizon, EC the true horizon, then the parallax is the difference between the angles SAB and SEC. The latter is the angle re * The Nautical Almanac is published annually, by order of the Lords Commissioners of the Admiralty, generally four years forward; in it are entered the sun's longitude, right ascension, declination; the planets' longitudes, latitudes, times of passing the meridian; the times of solar and lunar eclipses, together with those of Jupiter's satellites; the distance of the moon from the sun and certain fixed stars, at the beginning of every third hour; and in general the times when any remarkable appearances take place, being all computed for Greenwich time. quired, and is greater than the observed angle, for FEC EFA, AB and E C being parallel, But EFA FAS+ FSA, therefore = Consequently the correction for parallax, or for reduction of the observation to the earth's centre, must be additive. "Tables for obtaining the above corrections may be found in MR. BAILEY'S Astronomical Tables, &c., in the Requisite Tables, or in any modern work on navigation. 66 Example (from an Observation on Ship-board). Obs. alt. of the sun's lower limb (July) = 61° 13′ 5′′ Index error Apparent altitude Sun's semidiameter (see Table, p. 288) 17 = 61 24 3 Dip of the horizon, for an True altitude of the sun's centre When a "lunar distance," i. e., the distance between the sun and moon, or between the moon and a fixed star or planet, is required, the instrument is held in the plane passing through the eye and the two objects, the fainter object being observed by direct, the brighter by reflected vision. The inconvenience of measuring, with the sextant, angles between terrestrial objects, whose horizontal distance is that which is generally required, is, that the angles, being measured in planes parallel to the plane in which the eye and the two objects are situated, have to be reduced to their horizontal value, as explained in page 113. T When the altitude of a celestial object is to be taken on land with the sextant, an artificial horizon is used. Of these there are various constructions, all of which aim at presenting a reflecting plane parallel to the natural horizon, from which the rays of the celestial object may be reflected to an eye placed in the direction of the rays of reflexion. The angle measured in such a case is double the altitude of the object above the true horizon. Reflecting Plane Fluid Artificial Horizon. Among the various fluids used for the purpose of presenting such a reflecting surface, mercury has been found most useful. But during the observations its surface must be protected from agitation by the external air; for this purpose a roof-shaped cover is placed over the trough in which the mercury is contained, two plates of glass being fixed in the sides. These plates of glass should have perfectly parallel faces to avoid irregular refraction; but as this cannot be ensured exactly, two observations ought always to be made with the roof in reversed positions, in order to correct any error occasioned by undue refraction. Mirror for an Artificial Horizon. The mercurial reflector thus obtained presents an accurately level surface, but is frequently inconvenient and necessarily slow in its application. A more portable artificial horizon is one formed of a well-polished reflector, supported on three adjusting screws, the motion of which, |