from the center of the instrument. Note where the central colli- 

 mated horizontal cross wire intersects the rod. Set two targets, if 

 convenient, exactly 0.5 ft. above and below this middle position. 

 Turn the stadia adjusting screws, shown in Fig. 61, until the wires 

 bisect the targets so placed. Repeat the test at 200 ft. plus (c -f f), 

 at 300 ft. plus (c -H f ), etc. If the metric system is used, the test 

 targets should be spaced 0.5 m above and below the middle position 

 at 100 m plus (c + f ), let (c -f f ) be expressed as it may. 



Disappearing Stadia were devised in 1880 by Verplanck 

 Colvin, then N. Y. State Surveyor. When the stadia wires are 

 mounted on the same diaphragm with the regular cross wires, as in 

 cut 2, Fig. 62, there is some chance of using the wrong horizontal 

 wire for leveling or for observing vertical angles, particularly in the 

 dusk or under similar unfavorable circumstances. We mount the 

 ordinary disappearing stadia on the same reticle with the regular 

 cross wires but in a different focal plane, so that when the eyepiece 

 is refocused the field appears as in cut 6. It is not strictly 

 necessary, but most engineers prefer some sort of centering device 

 as in cut 7. These wires are commonly known as the St. Andrew 

 cross, and while they are accurately centered to the same alignment 

 with the principal cross wires, they do not, however, provide for so 

 great assurance in reading half-intervals, or vertical angles, so that 

 the arrangement shown in cut 5 is frequently specified. Occasionally 

 also a single vertical wire is added to the combination shown in 

 cut 7 to assist in plumbing the rod. For mining work the disap- 

 pearing stadia are commonly used to avoid confusion in the dark, 

 but where they are preferred in the principal focal plane the central 

 horizontal wire is distinguished by a set of diagonal wires, as in 

 cut 4. 



Inclined Stadia Observations require a double correction. 

 The fact that the sight is inclined demands that the observed dis- 

 tance shall be reduced to the horizon. If the rod is held vertically, 

 as it usually is, then it is not normal to the sight-line and too large 

 an interval will be read, which must also be reduced. The objec- 

 tion to holding the rod inclined and normal to the sight-line is the 

 judgment which must be thus delegated to the rodman, as well as 

 the complications which would arise in getting elevation by this 

 means. 



The interval, AH, observed, is greater than the true reading, 

 ;/. It will appear from Fig. 63 that AMr BMs-=v. For most 

 observations the interior angles at / and r approach %' d 

 enough so that BsM and ArM may be treated as R.A. tiia 1 



109 



