In case a horizontal base above or below the instrument is used, 

 and it is desired to reduce the reading to the horizontal distance, the 

 following formula, which can be easily derived, should be used: 

 (ab) C sin y cos <v 



"R 

 in which: 



H = Horizontal distance from instrument to base, 

 ab Base as erected, 

 C = Constant, 



y Angle between base and line of sight of instrument, 

 o> = Vertical angle at instrument. 

 Conclusions. 



The stadia ceases to be useful when the wires cut over the top 

 of the rod. Using only the half interval of 1:200, a 13-ft. rod 

 would be useless beyond half a mile. The superior advantage of 

 the filar micrometer lies in the fact that very much greater distances 

 can be estimated directly from the center of the instrument with 

 any convenient length of base. For traversing streams or trails, 

 where much of the field of view is cut off with brush or some kind 

 of impedimentia, any small portion of the rod that is visible may- 

 be used as a basis of calculation. Extremely long sights may be 

 taken without relying upon the rod-man since cairn signals can be 

 constructed, at known distances apart, and left standing for sub- 

 sequent back sights. * Such bases, 500 ft. in length, have been 

 used in estimating distances of 25 miles or more. The accuracy is 

 equal to that of the stadia, ranging between + 0.1 and 0.5 of \%. 



The Vertical Arc on this instrument is a segment of 120 

 with graduations on the face, or side, and a double vernier reading 

 to minutes. The attached magnifier has a metallic pointer which 

 can be used, while reading, as an indicator to superimpose the mark 

 indicating the whole number of degrees. 



The Striding Level, as in the standard model, is removable; 

 4^4 in. long and has a sensibility of 60" per each 2 mm division 

 engraved on the glass. 



The Control Bubble, with protection sheath and tangent ad- 

 justment, is considered indispensable, not only on this instrument but 

 on our No. 95 Frontier Model as well, which is a miniature repro- 

 duction of the standard model No. 90. Slight inclinations of the 

 planchette in the direction of the sight cannot affect the true value 

 of the vertical angle if the index of the vernier is brought into the 

 nadir, or hori/on, as the case may be, before the observation is taken. 

 If the striding level is broken or lost, the planchette may be con- 

 veniently leveled by the control bubble after being properly adjusted 

 to the sight line. It is 2 in. long and has a sensitiveness of 60". 



r the "Canada-Alaska Bounds 

 g. Rfc . May M, 1914. 



