T TROUGHTON'S LEVEL B B T turns, are of exactly the same construction as have been already described as used for supporting the Y level. The diaphragm is furnished with three threads, two of them vertical, between which the levelling staff may be seen, and the third, horizontal, gives the reading of the staff by its coincidence with one of the graduations marked upon it. Sometimes a pearl micrometer-scale is fixed on the diaphragm, instead of the wires. The central division on the scale, then, indicates the collimating point, and by its coincidence with a division of the levelling staff gives the required reading of this staff; and the scale serves the purpose of measuring distances approximately, and of determining stations nearly equidistant from the instrument, since at such equal distances the staff will subtend the same number of divisions upon the micrometer-scale. In selecting a level of Troughton's construction, and also in testing and adjusting the collimation subsequently, Mr. Gravatt's method, already described, is the best to be used; and, when the line of collimation is thus brought into adjustment, if the bubble be far from the centre of its run, the fault can only be remedied by the maker; but, if the bubble settle very nearly in the centre of its run, the instrument may be deemed a good one, and, the divisions on the glass tube which coincide with the ends of the bubble being noted, the instrument must be set up for use with the bubble in this position. The line of collimation is set perpendicular to the vertical axis, in the same manner as in the Y level, by means of the capstan screws, B, B, the bubble being made to maintain the requisite position, as above determined, while the intrument is turned completely round on its axis. MR. GRAVATT'S LEVEL. THIS instrument is furnished with an object-glass of large aperture and short focal length; and, sufficient light being thus obtained to admit of a higher magnifying power in the eye-piece, the advantages of a much larger instrument are m obtained, without the inconvenience of its length. The diaphragm is carried by the internal tube a, a, which is nearly equal in length to the external tube. The external tube T, T, is sprung at its aperture, and gives a steady and even motion to the internal tube a, a, which is thrust out, and drawn in, to adjust the focus for objects at different distances by means of the milled-headed screw A. The spirit level is placed above the telescope, and attached to it by capstan-headed screws, one at either end, by means of which the bubble can be brought to the centre of its run, as in the case of the Y level, when the line of collimation is brought to the proper level by Mr. Gravatt's method of adjustment, already explained. The telescope is attached to a horizontal bar in a similar manner to Troughton's level, but room is just left between the telescope and the bar for the compass-box. A cross level, k, is placed upon the telescope at right angles to the principal level 7, 7, by which we are enabled to set the instrument up at once with the axis nearly vertical. A mirror m, mounted upon a hinge-joint, is placed at the end of the level 1, 1, so that the observer, while reading the staff, can at the same time see that the instrument retains its proper position—a precaution by no means unnecessary in windy weather, or on bad springy ground. The telescope is attached to the horizontal bar by capstanheaded screws, B, B, as in Troughton's level, by which the line of collimation is set perpendicular to the vertical axis; and the instrument is set up upon parallel plates, as before described, for the Y level. Y CHAPTER VI. THE TRANSIT INSTRUMENT. REFLECTING instruments, from their portability and the promptitude and facility with which they may be used in all situations, and upon all occasions, are very useful instruments to the Surveyor. The Sextant with an Artificial horizon, and a good chronometer, forms an observatory of itself, with which the latitudes and longitudes of places may be determined to a certain degree of accuracy. In permanent observations the capital angular instruments are placed permanently in the plane of the meridian, and the measurements sought for by their aid are the exact times at which the observed objects pass the meridian, and their angular altitudes or zenith distances, when upon the meridian. The instrument with which the first of these measurements is obtained, is called a transit instrument, transit telescope, or merely a transit. Transits of portable dimensions, besides their use in small or temporary observatories, are found serviceable to the Surveyor, for determining, with the greatest possible accuracy, the true north point, and thence setting out a line in any required direction. The figure in the next page represents a portable transit. T, T is a telescope formed of two parts, connected by a spherical centre-piece, into which are fitted the larger ends of two cones, the common axis of which is placed at right angles to the axis of the telescope, to serve as the horizontal axis of the instrument. The two small ends of these cones are ground into two perfectly equal cylinders, called pivots. The pivots rest upon angular bearings or Y's. The Y's are supported upon the standards E and W, of which E may be called the eastern, and W the western standard; one of the Y's is fixed in a horizontal groove, on the western standard, so that by means of the screw S, one end of the axis may be pushed a little forwards or backwards, and a small motion in azimuth be thus communicated to the telescope.* The standards, E and W, are fixed by screws upon * The large transits in permanent observations have their Y's placed in two dove-tailed grooves, one horizontal, and the other vertical. By means of the latter, one end of the axis may be raised or depressed; but in the portable transit the same object is attained by turning one of the foot screws upon which the entire instrument rests. |