NAVIGATION 1 .

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lever on the under side of the quadrant, by which the glass maybe turned a few degrees on its axis, to set if parallel to the index-glass. The lever has a contrivance to turn it slowly, and a buttonto fix it. To set the glasses perpendicular to the plane of the instru-ment, there are tw o sunk screws, one before and one behind eachglass: these screws pass through the plate on which the frame isfixed into another plate ; so that bv loosening one and tighteningthe other of these screws, the direction of the frame with its mirrormay be altered, and set perpendicular to the plane of the instru-ment. There are usually three coloured glasses, K, two of whichare tinged red and the oilier green. They are used to prevent thesolar rays from hurting the eye at the time of observation. Theseglasses are set on a frame which turns on a centre, so that theymay be used separately, or together, as the brightness of the sunmay require. The green glass is particularly useful in observa-tions of the moon ; it may be also used in observations of the sunif that object be very faint. In the fore observation, these glassesare fixed as in fig. 1 ; but when the back observation is used, theyare removed to N. Each of the two sight-vanes, II, I, is aperforated piece of brass, designed to direct- the sight parallel tothe plane o! the quadrant. That which is fixed at I is used for thefore, and the other for the back, observation. The vane I has twoholes, one exactly at the height of the silvered part of the horizon-glass, the other a little higher, to direct the sight to the middle ofthe transparent part of the mirror. The limb of the quadrant isdivided from right to left inio ninety primary divisions, which areto be considered as degrees, and each degree is subdivided intothree equal parts, which are therefore of twenty minutes each : theintermediate minutes are obtained by means of the scale of divi-sions at the end of the index. The vernier, nonius, or subdivid-ing scale, contains a space equal to twenty-one divisions of thelimb, and is divided into twenty equal parts. Hence the differ-ence between a division on the dividing scale and a division onthe limb is one-twentieth.of a division on the limb, or one minute.■'The degree and minute pointed out by the dividing-scale, may beeasily found thus: Observe what minute on the-dividing-scale co-incides with a division onthelimb ; this division being added to thedegree and part- of a degree on the limb, immediately precedingthe first division on the dividing scale will be the degree and mi-nute required. Thus, suppose the fourteenth minute on the di-viding scale coincided with a division on the limb, and that thepreceding division on the limb to o on the vernier was 56*40';hence the division shewn by the vernier is 50° 54'. A magnifying-glass will assist the observer to read off the coinciding divisionswith more accuracy. The adjustments of Hadley’s quadrantconsist in placing the minors perpendicular to the plane of the in-•strument. The fore horizon-glass must be set parallel to the spe-culum, and the planes of the speculum and back horizon-glassproduced must be perpendicular to each other when the indexis at o.

Adj. I. To set the index-glass perpendicular to the plane of thequadrant: 1. Set the index towards the middle of the limb, andhold the quadrant so that its plane may be nearly parallel to thehorizon: then look into the index-glass; and if the portion of thelimb seen by reflection appears in the same plane with that seendirectly, the speculum is perpendicular to the plane of the instru-ment. If they do not appear in the same plane, the error is to berectified by altering the position of the screws behind the frame ofDie glass. 2. Another method is performed by the two adjustingtools, fig. 2, 3, which are two wooden frames, having two lines on•each, exactly at the same distance from the bottom. Place thequadrant in an horizontal position on a table; put the index aboutthe middle of the arch; turn back the dark glasses; place one ofthe above-mentioned tools near one end of the arch, and the otherat the opposite end, the side with the lines being towards theindex-glass; then look into the index-glass, directing the sight pa-rallel to the plane of the instrument, and one of the tools will beseen by direct vision, and the other by reflection. By moving theindex a little, they may be brought exactly together. If the linescoincide, the position of the mirror is right; if not, they must bemade to coincide by altering the screws behind the frame, asbefore.

Adj. II. To set the fore horizon-glass perpendicular to theplane of the instrument. Set the index too; hold the plane of;he quadrant parallel to.thc horizon; direct the sight to the hori-

zon, and if the horizons seen directly, and by reflection are appa-rently in the same straight line, the fore horizon-glass is perpendi-cular to the plane of the instrument; if not, one of the horizonswill appear higher than the other. Now if the horizon seen byreflection is higher than that seen directly, release the nearest:screw in the pedestal of the glass, and screw up that on the faitherside, till the direct and reflected horizons appear to make onecontinued straight line. But if the reflected horizon is lower thanthat seen directly, unscrew' the farthest, and screw up the nearestscrew till the coincidence of the horizons is perfect, observing toleave both screws equally tight, and the fore horizon-glass will beperpendicular to the plane of the quadrant.

Adj. III. To set ihe fore horizon-glass parallel to the index-glass, the index being at o. Set o on the index exactly to o or.the limb, and fix it m that position by the screw at the under-side;hold (he plane of the quadrant in a vertical position, and direct thesight to a well-defined part of the horizon; then if Ihe horizon *seen in the silvered part coincides with that seen through the trans-parent part, the horizon-glass is . adjusted ; but if the horizons do--not coincide, unscrew the milled-screw in the middle of the leveron (he other side of the quadrant, and turn the nut at the end ofthe lever until both horizons coincide, and fix the lever in thisposition by tightening the milled screw. As tire position of theglass is liable to be altered by fixing the lever, it will, therefore, .be necessary to re-examine it; and if the horizons do not coincide,it will be necessary either to repeat the adjustment, or rather to-Jind lire error of adjustment; or, as it is usually called, the index-error; thus: Direct the sight to the horizon, and move the indexuntil the reflected horizon coincides with that seen directly ; thenthe difference- between o on the limb and o on the vernier is theindex-error; which is additive when the beginning of the vernieris to the right of o on the limb, otherwise subtractive:

Adj. IV.'To set the back horizon-glass perpendicular to tharplane of the instrument. Put the index too; hold the plane ofthe quadrant parallel to the horizon, and direct the sight to thehorizon through the back sight-vane. Now if the rt fleeted hori-zon is in the same straight line with that seen through the transpa-rent part, the glass is perpendicular to the plane of the instrument:if the horizons do not unite, turn the sunk screws in the pedestalof the glass until they are apparently in the same straight line.

Adj. V. To set the back, horizon-glass perpendicular to the-plane of the index-glass produced, the index being at o. Let the.index be put as much to the right of o as twice the dip of the hori-zon amounts to ; hold the quadrant in a vertical position, and ap-ply the eye to the back-vane : then if the reflected horizon coin-cides with that seen directly, the glass is adjusted; it they do not -coincide, the screw in the middle of the lever on the other side ofthe quadrant must-he released, and the nut at its extremity turnedtill both horizons coincide. The reflected horizon will be invert-ed ; i. e. the sea will be apparently uppermost, and the sky low-ermost. As this method of adjustment is esteemed troublesome,and is often very difficult to perform at sea, various contrivanceshave been proposed to render this adjustment more simple. Someof these are the following: 1. In Mr. Dollond ’s method of adjust-ing the hack horizon-glass, an index is applied to it, by which itmay be moved so as to be parallel to the index-glass, when o onthe vernier coincides with o on the limb. When tin's is effected,the index of the back horizon-glass is to be moved exactly 9°from its former position, which is known by means of a divided'arch for that purpose;'.and then.the plane of the back horizon'glass will be perpendicular to the plane of the index-glass pro-"diiced. 2. In Mr. Blair’s method all that is required is to polishthe lower edge of the index-glass, and expose it to view. Theback horizon-glass is adjusted by means of a reflection from thispolished edge in the very same method as the fore horizon-glass isadjusted by the common method. To illustrate this, let RII1&fig. 5, represent a pencil of rays emitted from the object It, inci-dent on the index-glass I, from which it is reflected to the fore ho*rizon-glass II, and thence to the eye at E. By ibis double re'flection, an image of the object is formed at r. RT1 E representsanother pencil from the same object R, coining directly throughthe fore horizon-glass to the eye at E; so that the doubly reflectedimage r appears coincident with the object R itself, seen directl}'')When this coincidence is perfect, and.the object It so very dis'tant as to make the angle III II insensible, the position of the sp 6 "

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