The level rides on the pivots with its Y's. There Is a pin at each end, which drops into a fork at i, to hold the level safely and upright. This is completely seen at the left pier. At this end is the adjustment for setting the level tube parallel with the axis. At the other end is an adjustment for raising or depressing that extremity of the leveL The level should be very sensible and of the same curvature through out The graduation we have found most convenient is to have the principal divisions to 15" and the subdivision to 1'.5, numbered as unite and tenths, which, though erroneously, is briefly described by calling the units seconds of time. If this scale should be too fine for the level, a principal division to 30", and subdivisions to 3", but still numbered as unite and tenths, will bo found equally convenient. The riding level is generally applied to the instruments which are so large, and consequently the piers so high, that a man cannot apply the level safely while standing on the floor, and also to small instruments, of necessity, when they are clamped, as this is, to the pier. For a transit between atone piers which does not exceed five or six feet, we prefer a swinging level, which may be applied and read while standing on the floor.
When Troughton undertook, much against his will, to construct a ten-foot transit for the Royal Observatory, he adopted a very ingenious mode of uniting the cones and the telescope with the central sphere. The description will be found in the Phil. Trans.' for 1826, p. 423 : that part which treat, of the construction of the instrument is from Troughton's own pen. He also added four braces, to connect the telescope with the axis. We are not disposed to attach much value to this mode of connecting the axis and telescope, which, moreover, requires very accurate fittings. The braces are positively injurious, unless they are exactly and at the same moment exposed to the same temperature. It is said, indeed, in the memoir jnet mentioned, that when the antagonist braces were exposed to very different tempera tures, the instrument continued to preserve its form. If so, the expe riments simply show the centre-work to have been so strong that the braces could not disturb it, in which case they are merely useless. At Cambridge the braces were found to derange the instrument, and were consequently removed, to the great improvement of its steadiness.
There is no great difference of construction between different tran sits, except what we have already mentioned. It is desirable even for the smallest instruments that the supports should be of stone when they are not perpetually shifted about. The Y'a then are separate pieces fixed by screws to plugs let into the stone. For small transits the stone may be in one or three pieces, according to the size. When
practicable, the piers should be high enough and wide enough apart to let the observer stand or lie down between them. This saves perpetual meddling with the eye-piece, and the eye is leas strained. We have already remarked that the performance of a well-made transit depends rather on the permanence of its fixing than anything else. It is to the greater care bestowed on the foundations of largo instruments that much of their superior performance is to be attributed.
The principal use of a transit instrument is that of determining the exact moment when a celestial body passes the meridian of the place of observation. Now the meridian is a great circle which passes throdgh the zenith and the pole, and the instrument is adjuited when the line of sight • is a portion of the meridian during the whole rota tion of the telescope.
As in ell other instruments, the telescope is first to be adjusted for distinct vision. Put on a tolerably high power, and slide the eye-piece out and in till you see the wires sharply without straining the eye. Then direct the telescope to a bright star or a double star ; and if the Imago of it is distinct, the telescope is In focus. If not, release the screws at k, and draw the tube out or push It in until the image is as perfect as you can make it There is another opposite screw to L., and the exterior holes allow a little play. Some trouble and guesswork may be saved by making two slight scratches on the eye-plece where the eight of the wires and of the star are respectively most perfect, and drawiug the principal tube out or pushing it in thin quantity. The operation has succeeded, if, in viewing a slow-moving star, like Polaris, there is no shifting between the star and the wire which bisects it, while the observer moves his head laterally. This adjustment Is gene rally best made by the instrument-maker, and ss it is not liable to alter, we should prefer to have the telescope tube cut the proper length upon his responsibility, so that the position of the wire is permanent When this adjustment is completed, the telescope must be turned on some tolerably distinct object, which is to be bisected by the middle wire near the upper part of the field. If, on raising the telesoope, it is also bisected at the lower part of the field, the wire is perpendicular to the axis ; but if not, the tube is to be twisted without altering the focal length until the object comes half-way to bisection. The bisection is completed-by the azimuth-screw, when the object ought to be seen bisected at the top of the field when the telescope is depressed. One or two trials will suffice for this purpose, and then the screws at I: must be tightened.