The standards or A-frames in the ordinary plain theodolite are attached to the vernier plate, but not generally to the compass-box. The pivots of the transverse axis, which are made exactly equal in size, rest on coupled bearings on the tops of the standards, which are in construction made together, and therefore exactly alike. The transverse axis is not adjustable, as in the transit theodolite previously described; the standards have therefore to be adjusted to height in the manufacture by filing, with the application of a special striding level, until the transverse axis is brought permanently perpendicular to the vertical axis.
Fig. 188.—5-inch plain theodolite.
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439.—The vertical arc is fitted over the transverse axis; that is constructed with a turned flange to which the arc is firmly screwed. The arc is divided to 30′ and reads with a vernier to minutes. The vernier is fixed directly to the vernier plate, and reads generally with a microscope jointed on the transverse axis, but sometimes with a loose magnifier for economy. Divisions for difference of hypotenuse and base are occasionally divided on the back of the arc. The vertical arc has a clamp and tangent placed at the back, therefore this cannot be shown in the engraving. Along the bar above the vertical arc a stout plate is attached by screws. From this a pair of Y's with clips and eye-pins, as described for the Y-level, art. 192, supports the telescope.
The telescope is of the same construction as that described for Y-levels, with turned collars. The diaphragm is cross-webbed. For economy a simple cap is generally put to the telescope instead of the better plan of a ray shade. The principal level is fixed to collars fitted round the telescope, to which are attached one slot-piece for lateral adjustment of the level, and one screw-piece for linear adjustment by means of two capstan nuts. The level is placed under the telescope for compactness.
440.—The parts of the plain theodolite below the standards are the same as those already described for the transit theodolite, except that the vernier plate carries one level only at right angles to that of the telescope. The telescope is therefore set to zero by the vertical arc, and the two levels are then used as the pair upon the vernier plate of the transit. The means provided for the adjustment are the same as those of the Y-level, but the Y's are adjusted firmly by the maker by fitting them down upon the Y-plate in the manufacture.
441.—The plain theodolite, except where price is the first consideration, appears to be going gradually out of use, being superseded by the transit. It has had a long day since its first conception by Sisson about 1730. For 4-inch and 5-inch instruments the makers still find a small demand. The 6-inch is rarely enquired for. The plain theodolite cannot compete with the transit for perfect utility, but it holds the merit of less weight and of greater portability. The weights of the three sizes in general use are as follows:—
Weights of Plain Theodolites.
| Instrument. | Case. | Outer Case. | Tripod. | |||||
|---|---|---|---|---|---|---|---|---|
| 4-inch | 7 | lbs. | 7½ | lbs. | 3½ | lbs. | 8 | lbs. |
| 5-inch | 11 | " | 8½ | " | 4 | " | 9 | " |
| 6-inch | 17 | " | 10 | " | 5 | " | 11 | " |