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Optical Plumbing Assembly
Chaining Pin

Engineering Aid 3 - Beginning Structural engineering guide book
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Figure 11-43.-Types of tape clamp handles. marked circle reticle adjusted to be in line with the  vertical  axis  of  the  instrument.  After  the instrument  is  leveled,  a  sighting  through  the plummet  will  check  the  centering  over  a  point quickly. The advantages of the plummet over the plumb  bob  are  that  it  permits  the  observer  to center  over  a  point  from  the  height  of  the instrument  stand,  and  it  is  not  affected  by  the wind. The plummet is especially useful for work on  high  stands.  A  plumb  bob  requires  someone at  ground  level  to  steady  it  and  to  inform the  observer  on  the  platform  how  to  move  the instrument and when it is exactly over the point. With the plummet, the centering and checking is done  by  the  observer. Tape Accessories There is usually a leather thong at each end of a tape, by which the tape can be held when the full length is being used. When only part of the tape is used, the zero end can be held by the thong, and  the  tape  can  be  held  at  an  intermediate  point by means of a tape clamp handle, like those shown in  figure  11-43. When  a  tape  is  not  supported  throughout— that  is,  when  it  is  held  aboveground  between  a couple  of  crew  members—a  correction  must  be applied  for  the  amount  of  sag  in  the  tape.  To make this correction, you apply a certain amount of  tension.  Figure  11-44  shows  two  devices  for applying  a  given  amount  of  tension. 45.780 Figure  11-44.-Tension  scale  and  spring  balance. 11-41







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