46 



Plumbing and Centering Arrangements. 



It now remains to speak of several conveniences of the instrument under con- 

 sideration. By a simple mechanical contrivance the plumb-bob when suspended 

 from the instrument can be set immediately at any desired height. Jt is suspended 

 directly from the center of the instrument, and not from the tripod head. This 

 precaution shou 1 **. be taken *vith every instrument, since otherwise, when there is 

 difficulty in setting up an instrument, and the'legs are unsymmetrically placed, the 

 plumb-line will not pass through the center of the instrument. 



The instrument is provided with the shifting tripod, better known as the shifting 

 center, by means of which, when the plumb-bob of the instrument is within a fraction 

 of an inch over a point on the ground, it may be brought immediately over it, by 

 moving the body of the instrument on its lower level plate. This is probably the 

 greatest time-saving arrangement which modern makers have introduced in engin- 

 eers' transits. 



Shifting Center for a Transit with Three Leveling Screws. 



As made by C. L. Berger & Sons. 



In this device, as shown in the cut, the leveling screws rest in receptacles on the 

 piece made to slide on the tripod head. A clamp-nut, provided with a large flange 

 and handles, serves to secure this sliding piece to the tripod in any position in the 

 range of its lateral motion. The center-piece, bearing the hook for the plumb-bob, 

 is like that shown in the p. 179 cut; the milled-head, at its lower end, serving to fasten 

 it to the instrument; while its milled-headed nut, acting against the spiral spring, 

 fastens the instrument to the tripod. In use, the pressure of this spring needs to be 

 sufficient to take up the back lash or any looseness that may exist in the leveling 

 screws; but to secure the necessary stability of the instrument to the tripod, the clamp- 

 nut should be well fastened to the sliding piece. To operate the shifting center, both 

 the spiral-spring and the clamp-nut must be released slightly from their hold upon 

 the tripod and the sliding piece, when the instrument can be moved over the given 

 point on the ground. This device adds about 2 Ibs. to the weight of the tripod. 



c. L. B. 



