33

C—la

Face Right, Depression.

Face Right, Elevation.

Vertical Cibcle op Micrometer Theodolite No. 219. Note.—ln the above diagrams the circle is shown as fixed in one position, but it can be moved to any position by unscrewing the clamp and turning the circle by hand. Example 11. (See Diagram.) Zero-line almost coinciding with the left division-mark. Let b read .. .. .. .. .. 360° 00' 04" and f read .. .. .. .. .. 02 Error in run.. .. .. —2" Here the whole run must be applied to the b reading. The interval is again 2 seconds short of five complete turns, and must be increased, giving a correction g —2 to the b reading (since it is read backwards against the screw-head numbering). Corrected figures : b = 300° 00' 04" - 2" = 360° 00' 02" t = 02 - 0 = 360 00 02. Correction to the mean reading would be half the total run : 360° 00' 03" - 1" = 360° 00' 02". Example 111. (See Diagram.) Zero-line almost coinciding with the right-hand division-mark. Let b read .. .. .. .. .. 360° 04' 58" and f read .. .. .. .. 56" Error in run .. .. .. — 2" Again the interval reading is short of the five complete turns, but this time the whole run has to be applied to the f reading. A plus correction to the actual figures will increase the interval reading : — Corrected readings : b = 360° 04' 58" + 0" = 360° 04' 58" /= 56 +2 = 58. The correction to mean reading b^ f will be +1" : 360° 04' 57" +I" = 360° 04' 58". 64- f From the preceding examples it will be seen that the corrections to the mean reading -5* for run vary from 0, when the zero-line of the micrometer is midway between the division-marks (i.e., at reading 2' 30"), to half the total run when at extreme ends with opposite signs. Any intermediate reading can be corrected relative to the zero-line of the micrometer. When / reading is less than b the correction to -Ji will be minus from 0' to 2' 30" and plus from 2' 30" to s'. When /is more than b the signs wilLbe reversed. 5—C. IA.