THE THEODOLITE 



or geodetic, cadastral ami mining surveying, or pri- 

 mary and secondary triangulation, for latitude and 

 time observations and other classes of field astronomy, 

 and for a variety of other high class work, where 

 the most substantial and accurate construction is 

 required, the transit-theodolite is the instrument 

 preferred and adopted. 



The absence of the compass permits casting the 

 standards in a single solid U-shaped structure with its 

 broad base mounted directly over the flange of the vernier plate. 

 This method gives the instrument a rigidity impossible in other 

 types and the design provides for a most successful and symmetrical 

 combination, uniting strength with beauty of form. 



The size of theodolites and all transit instruments is technically 

 fixed, both in Europe and in America by the outside diameter of 

 the divided circle. The 4j' 2 -inch, the 5-inch and the 6-inch models 

 are of the so-called portable size that are carried about in a single 

 case. They are all designed for excellent service and hard use and 

 may be mounted either on the regular 4-scre\v base or upon our 

 own 3-screw model, at the discretion of the purchaser. At the end 

 of this chapter the respective weights are given in the specification 

 tables. 



The 7-, 8-, 10- and 12-inch models will not be carefully con- 

 sidered in this edition. The European 3-screw base and beveled 

 horizontal plates habitually furnished \\ith these instruments are 

 not yet popularly adopted for the smaller models. Fig. 73 on the 

 opposite page shows a No. 456 4,'^-inch model mounted on the 

 European 3-screw base, as we have madethemoccasionallyforGov't. 

 Engr's. operating in remote districts. The telescope is provided 

 with the cryptic focus and a magnification of ; .18 if erecting, or 

 either Ifii-i 20 it inverting. The circles read to 30", and the 

 weight of the entire equipment with extension leg tripod does not 

 exceed 9 kg. 



Open Telescope Bearings and the solid U-standards are 

 the two principal features which differentiate the theodolite from all 

 other instruments. Some of the smaller sizes are constructed with 

 closed bearings as in l-'ig. 74, but these are for use on general work 

 where the most compact construction is preferred and where the 

 compass is not regarded as an indispensable adjunct. 



In our lectures on adjustments under IX, p. 72, we emphasi/e 

 the necessity of concentric mounting in the objective, straight line 

 movement in the focusing device, a telescope whose geometrical 

 and optical axes are coincident and mid-way between the bearings, 

 and finally, journals that are spaced equi-distant from the center of 

 the vertical axis of rotation. Not all makers have been capable of 



