meeting these requirements, and geodetic engineers began long ago 

 to correct for residual collimation error, due to eccentricity of the 

 telescope, by taking two double sets of readings: first, with the 

 telescope erect, then inverted, then by removing it from its bearings 

 and, turning the instrument about, duplicate the system of observa- 

 tions with the telescope thus reversed. 



This implies the necessity for readily accessible bearings, as 

 shown in Fig. 79. In this case the axle hubs are always cylindrical, 

 resting in V-bearings or interrupted cylindrical surfaces. The caps 

 are raised or opened, the plungers of the opposite tangent screws 

 are released by winding up the little set screws at the end of each 

 spring house (see Fig. 81) when the telescope, with circle and tan- 

 gent screws, can be picked out of the standards. 



The nose-pieces at each side, against which the tangent screws 

 operate, must be interchangeable in fit. The illustration on p. 128 

 will also suggest the necessity for this. The tangent adjustment, 

 which is attached to the vertical circle, is used at will to center the 

 Control Bubble as with the Index Adjuster described on p. 96. 



Of the various requirements enumerated in the last para- 

 graph on page 129, the one which provides for bearings that 

 are equally spaced from the vertical axis of rotation, is dependent 

 for its permanency upon the capacity of the standards to resist 

 ordinary shocks and lateral strains. That the U-standards of the 

 theodolite are best adapted to preserve collimation for all distances 

 is a proposal that need not be submitted for debate. 



The ordinary truss standard of the ordinary transit, that is 

 shaped like the letter "A", must rely upon screws at the contact 

 point from the under side for its stability, and the ha/ard of 

 ordinary transportation, particularly in carrying cases with cup- 

 board doors, is sufficient to cause derangements that will come into 

 existence without being noticed. 



The standards of our Transits are reinforced at the contact 

 with the base plate with this purpose in view. It is not a mere 

 novelty in construction. The remarkable stability will be apparent 

 by grasping the journals one i f> . each hand and testing for side- 

 play or lost motion. This is a consideration of fundamental im- 

 portance that has never before received adequate treatment. 



The consolidated U-standard of our Tachy meters is the most 

 substantial construction that can be devised in one piece and still 

 contain a compass box. Eventually, when the American surveyor 

 comes to depend less upon the needle and will be satisfied with 

 those which are shorter, a still more substantial and elegant standard 

 can be designed. 



All standards of unsymmetrical design which violate the 

 mechanical sense, offend the artistic sense and transgress the com- 

 mon sense are diametrically opposed to the ethics of high class con- 

 struction. 



130 



