A transit instrument* placed in the plane of the meridian, of course affords the means of marking out at once a meridian line on the ground; the following short description, abridged from Dr. Pearson's "Practical Astronomy," explains the method of adjusting a portable transit approximately in this plane, and of verifying its position when so placed. 1st. The adjustment of the level, and of the axis of the telescope.-These two adjustments may be carried on at the same time; as when the level is made horizontal and parallel to the axis, the axis must be horizontal also.-Apply the level to its proper place on the pivots of the axis, and bring it horizontal by the footscrews of the instrument; reverse the level, and mark the difference as shown on the scale attached to it-half this difference must be corrected by the screw of the level, and half by the footscrews, which operation will probably want repeating-if by previous observation, the level has been ascertained to be correct, the foot-screws alone must be used in the correction, and if on reversing the instrument in its Ys, the level is still correct, the pivots of the axis are of equal size; if not, the instrument should be returned to its maker as imperfect. 2nd. The next object will be to place the spider lines truly vertical, and to determine the equatorial value of their intervals. Suspend a thick white plumb-line on a dark ground, at a distance from the telescope; then the middle wire may be made to coincide with it to insure its verticality, and if a motion in altitude be given to the telescope, and the coincidence continues unaltered by change of elevation, the axis has been truly levelled. The equatorial value of the intervals between the wires, may be determined by counting the time in seconds and parts occupied by the passage of an equatorial star over all the intervals, taken separately and collectively, by several repetitions on or near the meridian. If the star observed has any declination, the value of * In an Observatory, the principal uses to which a transit is applied, are the obtaining true time, and the determination of right ascensions—very excellent directions for using and adjusting a portable transit for the determination of longitudes, &c., drawn up by Mr. Airy, will be found in the Narrative of the North American Boundary, by Major Robinson, from which one example is given at the end of this chapter, to show the form there adopted for recording transit observations. an interval obtained from its passage may be reduced to its equatorial value by multiplying the seconds counted, by the cosine of the star's declination; before this method can be used, the telescope must have been placed nearly on the meridian. 3rd. Collimation in azimuth.-When the preceding adjustments have been made, the telescope should be directed to a distant object, the middle spider line brought to bisect it, and the axis then turned end for end. If, after this reversion, the same point be again bisected by the wire, it is a proof that a line passing from the middle spider line to the optical centre of the object glass is at right angles to the axis of the telescope's motion. But if, after this reversion of the axis, the visible mark be found on one side of the middle line, half the error thus found must be corrected by the screw which moves the Ys in azimuth, and the other half by the screw for adjusting the wires; several reversions must be made to ensure accuracy.-The verification of this adjustment may be proved by the passage of the pole star;-note the time at the preceding and at the middle wire, then reverse the axis, and note the passage over what was the preceding, but is now the following wire; half the difference of the intervals before and after reversion, will show how much the position of the centre wire has been altered by reversion. 4th. Collimation in altitude.—When the telescope is directed to the pole star at the time of its crossing the meridian, or to any well-defined distant point by daylight, read the vernier of the altitude circle, while the bubble of the level is at zero. The axis of the telescope must then be reversed, and the horizontal line again brought to bisect the star; and when the bubble is made to stand at zero, as before, the reading of the vernier must be again noted; half the sum of these readings will be the true altitude; and half the difference, the error of collimation in altitude. This error may consist of two parts: the spider line may be out of the optical centre of the field of view; and the level (supposing it previously adjusted to reverse properly in position) may not be in its true position as regards the zero of the circle's divisions; half therefore of the error arising from the half difference of altitudes must be adjusted by the screws carrying the spider lines, and the other half The screw that alters the level. 5th. The last and most difficult of all the adjustments, is that by which the instrument is placed in the plane of the meridian of the place of observation. There are many modes of accomplishing this, both by direct and indirect means; but the most convenient and most generally practised are those in which a circumpolar star is employed; or in which two circumpolar stars, differing little in declination, but nearly twelve hours in right ascension; or in which two stars, differing considerably in altitude, and but little in right ascension, are successively observed; but in whatever way the adjustment may be made, the clock that gives the times of transit must have its rate previously well determined. The approximate position of the instrument may be ascertained by calculating the solar time of the pole star's passage over the meridian for any given day; and then the telescope levelled and pointed at it, at the computed time, will require but little adjustment. Subsequent observations of circumpolar, or of high and low stars, will gradually rectify the position, provided all the adjustments previously directed, continue unaltered for a sufficient length of time; and a meridian mark, capable of adjustment, may be placed at a convenient distance north or south, until their places are definitely fixed by some of the following methods. At 95.49yards from the object end of the telescope, one inch will subtend l' or 60′′, and a scale may be made accordingly, varying of course inversely as the distances; so that when the transit is found to be any number of seconds, say thirty, too much to the east or west, a corresponding distance on the scale shows how much the instrument is to be moved in azimuth, by the proper screws, to effect the correction required. Method 1st.-By a circumpolar star. Where a = azimuthal deviation in seconds at the horizon, t = the time at upper transit, tat lower passage, L= the latitude, the declination : by multiplying by 15, a is converted into space if required. |
