The Averill Stadiagraph 



While considering the general subject of the stadia it may 

 be a matter of general interest to note here that C. K .V. orill, of 

 Yonkers, N. Y., has recently introduced a special protractor for 

 the rapid mechanical reduction of notes in plotting stadia topog- 

 raphy. 



It consists of the major portion of a 10-in. circle graduated 

 into degree-spaces, and a 10-in. radial straight edge divided to ' n, in. 

 The theory of its construction is based upon the well known for- 

 nuihe governing the computation of the stadia tables as given in the 

 appendix of this book. 



Figr. 67 



In the En?. News, Jan. 22, 1914, it says, "The pivoted scale, 

 KK, is the scale of the map and can be changed. This scale is used 

 to determine the horizontal distance from the transit station to the 

 stadia point by setting its graduated edge on the recorded vertical 

 angle reading, on the arc scale A, and mo\ ing theslidingscale, HH, 

 until its graduated edge intersects the edge of the pivoted scale 

 at the recorded rod reading. The intersection of HH and the 

 straight-edge is the point required. No great a.vurac\ is observed 

 in setting the graduated edge of the pi\oted scale on the arc scale, 

 A, as the square of the cosines vary but little for a few minutes of 

 angle. 



"The graduations of the sliding scale, HH, are determined by 

 the scale of B, which is an assumed scale based on the sines of dou- 

 ble the recorded vertical angles, so!\ ing the triangle for h = !_. Kl 

 sin 2 v. Placing the graduated edge of HH on the rod reading of 

 the straight-edge scale, the in?. Juated edge of 



II ami HH \\ill give the difference in elevation on HH for any 



117 



