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since it frequently happens, and not improperly so, that their adjustments constitute a part of the examination which all mates and midshipmen must undergo before obtaining a lieutenant's commission; which is the golden step to promotion in the Royal Naval Service of His Majesty.

ADJUSTMENT I.

To Set the Index-Glass, or Moveable Reflector, perpendicular to the Plane of the Sextant, &c.

47. Move the index to about 60:, viz., to near the middle of the arc or limb. Hold the sextant with its face up, the index-glass being next to the observer, and the arc, or limb, turned from him; and keep it so that its plane may be nearly parallel to that of the horizon. Direct the sight, in an oblique manner to the index-glass or moveable reflector ;(this is frequently called the speculum ;)—then, if the reflected limb seen in the glass be exactly in the same plane, or unbroken arc, with that seen by direct vision, the index-glass is truly perpendicular to the plane of the instrument. But, should the reflected limb appear to be raised above, or depressed below the plane of the real limb, the glass is not perpendicular: in this case, the screws at the back of the indexglass must be gradually, but most cautiously, turned till both limbs appear to be in the same plane, or to form but one continued arc. In the case of a quadrant, the index is to be set about at 45%, then proceed as above. Here it may be noted, that the present adjustment is more applicable to the quadrant than to the sextant: because, in all wellmade instruments of the latter denomination, the index-glass is rendered so very secure by the maker, that it can never lose its perpendicularity, so long as the sextant is handled in a proper manner.

ADJUSTMENT II.

To Set the Horizon-Glass, or Fixed Reflector, perpendicular to the Plane of the Sextant, &c.

48. The index-glass, or moveable reflector, being adjusted as above; set the O on the vernier or dividing scale of the index to O or zero on the limb of the instrument; and make the coincidence of these points quite perfect by means of the tangent-screw, using for this purpose the magnifying lens ;-when perfect, clamp the index by means of the screw at its back. Hold the sextant with its face up, or so that its plane may be parallel to that of the horizon; look through the sightvane, or through the socket which receives the telescope, and direct the sight to the horizon-glass, or fixed reflector; then, if the reflected horizon appear to be in the same uniform plane, or continued straight

line with that seen by direct vision, the horizon-glass is truly perpendicular to the plane of the sextant. But, should the horizon seen by reflection appear to be either above or below that seen directly, the upper adjusting-screw at the back of the horizon-glass is to be carefully turned, till the coincidence of the reflected and real horizons is quite perfect. In making this adjustment for a quadrant, it is to be observed, that if the horizon seen by reflection be higher than that seen by direct vision, the screw which is nearest to the glass in the pedestal is to be eased, and that which is farthest from it to be screwed up, till the two horizons appear to be in the same plane. But, if the reflected horizon be lower than that seen directly, the screw which is farthest from the horizon-glass is to be eased, and that which is nearest to it screwed up, till the coincidence of the reflected and real horizons appear to be quite perfect: taking care, however, to leave both screws equally tight.

49. The above adjustment may be conveniently made in the following manner, viz. :-Screw the telescope into its place; adjust it to distinct vision, and turn the tube or eye-part thereof until two of the cross wires are parallel to the plane of the sextant. Move the index, only loosely clamped, to about zero on the limb: `arrange the shades so that one dark glass may intervene above, and another below the fixed reflector. Hold the instrument with its face up, and direct the sight, through the telescope, to the sun, when its altitude is not more than about 20 or 30 degrees, or when it is not so great as to inconvenience the observer; taking care to hold it so that its plane may be parallel to the horizontal diameter of the sun. Move the index slowly, by hand, backwards and forwards; then, if the reflected image of the sun pass exactly over the face of the real sun, the horizon-glass is perpendicular to the plane of the sextant. But should the reflected sun appear to pass a little above or below the upper or the lower points of the real sun's disc, the glass is not perpendicular to the plane of the instrument, and therefore it must be adjusted, as directed in Article 48.

ADJUSTMENT III.

To set the Horizon-Glass, or Fixed Reflector, parallel to the IndexGlass, or Moveable Reflector.

50. The two preceding adjustments being completed, set the O on the vernier or dividing scale of the index, to O or zero on the limb clamp the index in this position, and make the coineidence of the points perfect by means of the tangent-screw; using for this purpose the microscope or magnifying lens; screw the telescope into its socket, adjust it to distinct vision, and turn the tube or eye-part thereof, until two of the cross-wires are parallel to the plane of

the sextant. Raise the socket of the telescope by means of the milled screw at back of the collar, until the field of view of the telescope is fairly bisected by the line which separates the silvered and the transparent parts of the horizon-glass. Hold the sextant in a vertical position; that is, with its arch or limb downwards. Look through the telescope, and direct the sight to the horizon: then, if the reflected horizon, and that seen by direct vision be exactly in the same plane, the horizon-glass is truly parallel to the index-glass. But, should the horizons appear to be broken, or not to be in the same continued straight line, the lower adjusting-screw at the back of the horizonglass is to be very carefully turned, till the coincidence of the two horizons appears to be perfect.

It may be right to observe that the adjusting-screws are placed differ. ently in different instruments, according to the fancy of the makers. In sextants of Berge's construction, the screw for the above adjustment is placed towards the lower part of the back of the horizon-glass; and that for the preceding adjustment towards its upper part.

In making the present adjustment for a quadrant, if the horizon seen by reflection does not coincide with that seen by direct vision, ease the milled-screw at the back of the fixed reflector, and turn the nut at the end of the lever till the two horizons appear to be in the same continued straight line; then, fix the lever in this position by tightening the milled-screw, In tightening this screw it frequently happens that the coincidence of the two horizons becomes imperfect; in this case, the adjustment is to be repeated until both horizons exactly coincide.

51. The above adjustment may be very correctly made in the following manner, viz.: Arrange the shades so that one dark glass may intervené above, and the other below the fixed reflector: hold the sextant in a vertical position, as before, direct the sight to the sun, through the telescope; ease the clamp of the index, and then move the latter gently, by hand, backwards and forwards, so as to cause the reflected image of the sun to pass up and down, in a vertical manner, over the face of the real sun. Then, if the reflected sun pass so exactly over the true sun as not to project beyond either its west or its east limb; that is, either to the right or left, the fixed reflector is truly parallel to the moveable reflector. But, should the reflected image project anything either to the right or left of the real sun, the adjusting-screw must be carefully turned, till the two suns appear to pass over each other in a direct vertical manner.

ADJUSTMENT IV.

To make the Line of Collimation parallel to the Plane of the
Sextant.

52. Note.-The Line of Collimation is an imaginary straight line joining the centre of refractions of the object-glass of the telescope and the centre or middle point between its parallel wires; at which central point the contact of the limbs of the two objects must be made in the act of taking a lunar observation.

The mode of making the present adjustment is as follows, viz. :— Screw the telescope into its place, adjust it to distinct vision, and turn the tube, or eye-part thereof, until two of the cross-wires are perfectly parallel to the plane of the sextant. Select two celestial objects whose angular distance is not less than a right angle: but, for this purpose the sun and moon are the fittest and most eligible objects, particularly when they are so situated in the heavens that the arc comprehended between them is nearly equal to a third part of the whole zodiac; that is, when their distance does not differ much from 120 degrees. Turn down all the coloured glasses, or shades that are above and below the fixed reflector, except one of the darkest, or deepest red, which is to be left standing betwixt the fixed reflector and the index-glass, so as to protect the eye of the observer from the effects of the solar rays. Look through the telescope; direct the sight to the sun, and cause its darkened image, by moving the index forward, to touch the moon. Clamp the index, and make the contact of the limbs, by means of the tangent-screw, as perfect as possible at the wire nearest to the plane of the sextant: then, without a moment's loss of time, bring the point of contact (taking great care not to move the index) to the other wire, or that which is furthest from the plane of the instrument. Now, if the contact appears to be perfect at this wire, the axis of the telescope is truly adjusted; that is, its line of collimation is perfectly parallel to the plane of the sextant. But, should the limbs of the objects appear to be either separated, or to partly cover each other, one of the screws of the collar in which the telescope is fixed must be eased, and the other screwed up, until the contact of the limbs appears to be perfect at both wires.

53. Instead of meddling with the screws of the collar, which should never be touched, except in extreme cases, the following method may be adopted, viz. :-When the contact is made at the wire nearest to the plane of the sextant, let an assistant note down, per watch that shows seconds, the exact moment of contact, and the corresponding angular distance; then, bring the point of contact to the other, or distant wire, and make it perfect by means of the tangent-screw; the assistant not

ing down, as before, the exact moment of its being perfected, and the value of the corresponding angular distance. Now, the difference of the times shows the interval between the moments of contact, and that of the corresponding angular distances, shows the approximate error in the line of collimation: which error is to be corrected on account of the change of distance, during the interval in the following manner, viz. Reduce the time of observation to the meridian of Greenwich: with which enter the Nautical Almanac, and take out, amongst the distances for the given day, the proportional logarithm answering to the hour which immediately precedes and follows the Greenwich time; to this add the proportional logarithm of the interval between the moments of contact; the sum will be the proportional logarithm of a correction; which, being subtracted from the difference of the angular distances when the limbs appear to be separated at the distant wire, or added thereto when they overlap or partly cover each other; the result will be the correct value of the error in the line of collimation. Now, this being known, its absolute effect on any other lunar distance may be readily found by means of Table XXIII.; as thus, enter that Table with the error, or "inclination of the line of collimation," at top, and the observed distance in the side column; in the angle of meeting stands a correction which is always subtractive from the observed distance and which may be used afterwards as a constant quantity to be applied to all lunar distances taken with the same instrument; so long as the error or inclination of the line of collimation, remains unaltered. 54. In sextants of Berge's construction, the above adjustment very rarely, if ever, becomes necessary; because in those, the collars, or sockets for the telescope are so well fortified, and so firmly supported that the parallelism of the line of collimation must always remain perfect so long as the instrument does not receive some violent shock: for, the line of vision is so well guarded, that nothing but foul play, such as an unlucky blow, or a fall from some height, can possibly turn the axis of the telescope from the true plane of the sextant.

To find the Index Error of a Sextant.

55. When a sextant is duly rectified agreeably to the four preceding adjustments; it is evident that it will then be perfectly free from errors, and therefore in a fit state for either measuring the angular distances, or for taking the altitudes of the heavenly bodies. But, since the adjusting screws are liable to be injured by much turning or screwing; and, moreover, since frequent adjustments have a tendency to derange the correctness of the best instrument; it is, therefore, highly advisable to tighten all the adjusting screws at once, when the sextant is perfect, and never to touch them again except in cases of absolute necessity;

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