If the rod is held inclined, however, there will be a slight error 

 in distance due to the fact that the central wire does not intersect 

 the rod at a point directly over the station. 



The "Interval" or "Stepping" Method of Determining 



Elevation 



The only references we have noticed setting forth this method 

 were published in the Eng. News, N. Y., April 28, 1910 by K. M. 

 Douglas; Sept. 1, the same year by A. F. Meyer, and in En%. and 

 Cont., Chicago, Feb. 18, 1914, by H. H. Edgerton. 



By reference to Mr. Meyer's cut, which is the upper portion of 

 Fig. 63, the several stadia intervals are to be laid off from the hori- 

 zon against "stepping points" in any visible back-ground, above 

 or below the instrument, until the rod appears in the field of view. 

 Let: 



ab = difference in Kiev, between H. I. and Sta. on hi! 

 c reading of upper w ire at n intervals 

 cd -- rod interval I 

 cd cos "v true interval 



Tc = 100 (cd cos v), nearly: 

 cTa n (34' 23") if K 100 

 ab - (Tc sin cTd) he; 



n (cd) be, approximately. 



n (cd) will be suffi- 

 ciently accurate for most 

 purposes up to n 3 

 -r Beyond this apply Mey- 

 i er's Rule as follows :- 



When more than 

 three intervals ate read, 

 reduce n (cd) or nl by 1 % 

 of itself for each interval 

 above three. 



Fig. 64, being an 

 adaptation from Mr. 

 Edgerton'sarticle, shows 

 the original field, A, 

 which is in the horizon. 

 In field B the telescope 

 has been raised so that 

 the lower stadia wire 

 | cuts the original horizon 

 point; then the telescope 

 is raised successively, by 

 steps equal to the stadia 

 interval, until the upper 

 stadia wire cuts the rod, 



Fig. 64 as in field D. The sta- 



dia interval is finally 



found, approximately, as in field E, setting one wire at any conven- 

 ient foot mark. We then have a total intercept of n 1 some overlap. 



112 



