The foregoing description has been confined to a form of theodo lite which is not in ordinary use, though from its simplicity and power it is well adapted to the purpose of explanation. The common theodolite is generally carried by a pair of parallel plates, fixed on a three-legged staff. The lower of these circular plates is screwed upon the staff, and has an aperture above the screw. The upper plate has a strong descending shank which primes loosely through this aperture. A button of a eplierical form is fixed on the end of the shank, the cur vature uppermost, and rubs against the nuder surface of the lower plate, which is dome-shaped to fit it Four strong screws pass through the upper plate and abut with their lower ends against the lower plate. When the screws are turned the plates are separated until the button and the spherical surface on which it rubs are brought into squeezing contact. To level the theodolite, set the levels each parallel to a diagonal pair of screws of the parallel plates. Then screw one pair until you come to a bearing, and by releasing one screw and !crewing up the other, but not very tight, set the corresponding level horizontal; leaving this pair and taking hold of the other pair set the second level also right, and if the first level is deranged, as it probably will be a little, restore its position by screwing up the proper screw. Turn the telescope half round and correct the error, half by the parallel plate-screws, and the other halt by the level-adjustments them selves. It is desirable that, when the final odjustmcnt is made, the screws should bite pretty hard, otherwise there is a great chance that the upper plate will turn a little during the observation. This objec tion would seem fatal to the use of parallel plates where great nicety is required; they are however very convenient and of very ready use, and perhaps if the screws are strong and the observer is careful to give the telescope three or four turns round in the direction he means to observe, before starting, and always to move the telescope the same way, serious error may be avoided. The first object observed should always be observed at the end of the service, in order to see whether there has been any change in the original position. If one of the screws rest in a notch, perhaps the tendency to twist may be wholly overcome.
Another contrivance which is to be found in almost all theodolites is much more objectionable. The surveyor wishes to save himself addition or subtraction, and requires an adjustment by which he can turn the whole circle about and bring the telescope upon the first object, the verniers being previously set to zero. There is, therefore, a motion with a clamp and tangent-screw for this purpose, which, a-s the clamp has usually a very short bearing, is particularly liable to yield, and so to destroy all accuracy. To remedy this unnecessary evil, a second or watch telescope, as it is called, is attached to this part of the instrument, and brought to bear upon a well-defined object. Any motion or wriggling of the zero-clamp is betrayed by the watch telescope, and when an angle is taken it must be first ascertained whether the watch-telescope keeps its position, and the position if dis turbed must be restored to the zero-tangent screw, before the observa tion is finally made. In some theodolites made for the Indian survey,
under the direction of Colonel Everest, the zero and slow-motion clamp take the form of a repeating-table, and may be so applied. It would be safer to have this motion made considerably heavier than in the pat terns we have seen, and if the instrument is likely to fall into clumsy hands the wateh-telescope might easily be added for greater caution. Such a theodolite would, so far as we'can judge, have no limit to its accuracy, except that depending on the diminutive telescope.
For many purposes of surveying it is desirable that the telescope should allow of being considerably elevated or depressed, and that means should be given for measuring this angle with considerable accuracy. A circle, or portion of a circle, is then fixed upon the tele scope axis, and the necessary verniers and level may be secured by a tail-piece or otherwise to the support. If the vertical angles are to bo measured as accurately as the horizontal angles, the instrument becomes an altitude and azimuth circle. [CIRCLE.] But such instruments are rarely applied to the measurement of terrestrial angles. The direction of the meridian was determined in the Ordnance Survey by observing Polaris at its greatest elongations E. and W., and taking the middle of the two readings for the direction of the north. hence, the telescope required all the transit adjustments except that for azimuth [Tn.saerr], and was considerably elevated above the circle. Though the results were upon the whole satisfactory, yet we greatly doubt the prudence of ascertaining this fundamental and delicate point from such an instru ment, or of risking the steadiness of the telescope supports by raising them so much above the body of the instrument. It would have been better, we conceive, to have determined the direction of the meridian by a series of careful transit observations, using more optical power with greater steadiness, and to have kept the theodolite to its proper office, that of measuring horizontal angles, greatly reducing the height of the telescope supports. The great theodolite had originally a semi circle fixed to the axis of the telescope, for measuring altitudes and depressions. This has since, very properly, been removed, and a whole circle substituted.
Where a theodolite is merely used for surveying, the telescope requires only a moderato vertical range. Mr. Troughton fixed a por tion of a circle (which may be more properly called a slice than a sector) to one or two of his 12-inch theodolites, and this construction is often found in other makers. The telescopo is thus kept lower, the Instniment is firmer, and the larger radius gives the portion of the circle a seeming advantage over the entire circle of smaller radius. There is, however, something very unsatisfactory in a portion of a circle, and we should prefer a sort of compromise, giving the supports such an elevation as would allow a vertical circle of about half the dimensions of the horizontal circle. If the direction of the meridian is to be determined by this instrument, the supports must be at least so high as to see or above the latitude of the place, and the vertical circle may be increased accordingly.