SECT. III.—.ficcount of M. Duconi's C:ylindrical ficial Horizon.
The instrument consists of two parts, one of which is a copper disk, six inches in diameter, with three feet. The second part is a cylindrical cover or drum, which performs the part of the glass roof in the com mon horizons for sheltering the fluid from the action of the wind. From the middle of the first part, or copper disk, there rises a hollow cylinder of white iron inches high, and 2.1 inches in diameter. Upon this cylinder, which is open at top, there is placed a small round disk of white iron, (or of boxwood, where mercury is used,) which goes into the top of the cy linder, but is prevented from descending by a ledge on which it rests. This disk contains the mercury, wine, or prepared syrup, which is employed. These cylin ders are adjusted in such a manner, that the surface of the fluid is exactly 21- inches above the first disk. On the copper disk are fixed two brackets, to which is fastened the cylindrical roof or drum. This drum, which is made of white iron, is six inches in diameter, and 21 wide, and is so placed that its centre is in the surface of the fluid in the round disk. In the middle of the width of the drum, there are two bands of white iron, perforated by two circular openings diametri cally opposite to one another, and an inch in diameter, the one for letting in the incident rays, and the other for letting out the reflected ones. They have a cir cular motion by a rack and pinion on the surface of the drum, for the purpose of being adjusted to the height of the sun or the star.
When there is not much motion in the air, two small funnels, or truncated cones, are placed in the small tubes in the circular apertures, and these have the effect of protecting the fluid surface from every agitation. When the wind is considerable, the fun nels are kept on, and a small glass with parallel faces is placed at the end, by which means the incident rays are admitted; but if the wind is very high, the funnels are taken off, and a piece of wire gauze is placed in the tube. This permits the external air to be in regu lar communication with the internal air, which is fa vourable to the accuracy of the observation.
Among the various contrivances for artificial hori zons as applied to the sextant, one of the simplest is that shown in Fig. 16, which is a view of part of a sextant constructed by Mr. Troughton. A small level EL is fixed between the index glass E and the horizon glass D, and there is a hole in the side of the brass tube containing the level through which the bubble can be seen by the observer at E, by reflexion in the horizon glass, as shown in Fig. 17. where a line r is drawn across the glass where the contact is to be made, at the same time that the image of the bubble is to be bisected by the same line. This apparatus is said to have given the altitude within 5 minutes of the truth.
46 This telescope, represented by AB, Plate CCCCLXXXVII. Fig. la, consists or three parts; viz. 1st, The eye-tube AE. to the lower side of r. hich a spirit-level x is attached by the screws o, p, pass ing through the extremities C and I) of the frame of the level tube; 2d, The object tube FB, which is at tached to the sextant by the screw at y; and, 3d, The middle, or connecting tube EF, represented separate ly by GH, Fig. 19, or which the part EH enters the
object tube, at F, and the part EG is screwed into the eye tube at E by means of the screw Eli, and thus brings the small glass G into its proper place at b near the field of the telescope. The reduced diameter of the part GK permits the upper side /c 1 of the level tube to enter 1-ath of an inch within the lower side of the eye tube, and thus brings the bubble, seen di rectly through the eye glass at A, as near as possible to the field of the telescope. In the centre of the field two cross hairs of silk intersect each other at right angles, the one horizontal and the other vertical; and the point of their intersection is adjusted exactly into the line of vision through the telescope by means of the screw nails e, (1, acting on the diaphragm, the edge of which, seen at e, is filed quite thin on the far ther side, for the purpose of more easily admitting the direct light of a lamp through the aperture, a b, to illuminate the cross hairs at night. is q r, and s t, are rectangular apertures in the frame of the level tube, and I r is a rellecior placed below s t to illumi nate the spirits, and to show more distinctly the posi tion of the bubble. The lines f and g, arc painted on the level tube at opposite extremities of the bubble, when it is in the middle; and, as the level is applied so that the linef is placed at the focal distance of the eye glass, the eye end of the bubble can be distinctly seen at f, and at 1-3d of an inch on either side of it. When, therefore, k 1, the upper side of the level tube is adjusted parallel to A13, the line of vision through the telescope, if the eye end of the bubble be observed, and kept at f, the line of vision AB must then be truly horizontal, or parallel to the horizon. In order to take the altitudes at sea by a quadrant or sextant, fur nished with this telescope and level, which may be made capable of distinguishing lel", the observer should hold the sextant, as usual, in a vertical plane, passing through the celestial object whose altitude is required, the telescope being horizontal. and then bring the reflected image of the sun, moon or star, into the field by the motion of the index on the limb of the instrument, which, after sonic experience, he trill generally be able to do upon the first or second trial. When the celestial object is thus brought into the field, and the near end of the bubble seen at f in the level tube, the observer should clamp the index on the limb, and, by means of the tangent screw, while the near end of the bubble is kept at f, bring the lower limb of the observed object to touch the horizontal hair passing through the centre of the field; the re quired altitude of the lower limb of that object, affect ed only by refraction, will then be found, as usual, on the limb of the sextant.