In using the formula for H and V it should be born in mind 

 that (c -f- f ) cos v will not differ materially from (c -f f ) up to 5 

 elevation, and that (c -f f ) " v is negligible for ordinary topog- 

 raphy up to this limii. With our cryptic focusing telescopes 

 (c + f ) is so reduced that this limit of negligence can be extended 

 to 9 or 10. 



For reading inclined observations, the center mark, M, should 

 be as high above the peg as the instrument is above its station but 

 this is frequently averaged at 5 or b}i ft. depending upon the 

 stature of the observer. For greater accuracy the H. I. Plummet, 

 illustrated and described on page 176, will be found convenient. 



One method of overcoming complicated formulae for H is to 

 place vertical stadia wires in the telescope, as in cut 3, Fig. 62, and to 

 have the assistant hold the rod steadily in a horizontal position, 

 This will give the exact inclined distance which can be reduced 

 directly to the horizon by the cosine of the vertical angle. The 

 vertical stadia are occasionally specified for levels to be used in 

 this way. E. M. Douglas, U. S. G. S., proposed sometime ago* 

 that the stadia wires should be mounted on a revolvable diaphragm 

 for this purpose, but the idea has not been generally adopted for 

 fear the collimation adjustment might be disturbed in the grocess. 

 The better and cheaper plan, lor levels would be to adopt the 

 diaphragm, as in cut 5, Fig. 62. 



Stadia Reduction Tables contained in the Appendix of this 

 publication are widely used in preference to diagrams or slide 

 rules. That part of the compilation up to and including 30 of 

 altitude was computed by Arthur Winslow for the 2nd. Geol. 

 Surv. of Pa., 1874-87, and that from 30, up, by H. N. Evanson of 

 Pittsburgh, some 20 years ago. We add the (c -f f ) corrections 

 for .40 and .60 ft. in each case to facilitate work with the lower 

 constants of our alidades and our 4 ^2 and 5-in. Tachymeters. 



A. P. Davis, Ch. Eng. U. S. Irr. Dept., and C. G. Anderson 

 of the U. S. G. S., have also computed tables for special work in 

 their respective departments, so that desired results can be obtained 

 by addition instead of multiplication. 



The Reduction Tables on page XXVII in the Appendix are] 

 taken from the Army Manual. They are intended also for the 

 reduction of the inclined measured distance to vertical and hori- 

 zontal components, but in order that the reduction shall be free 

 from error it is recommended that the rod must be inclined until 

 normal to the line of sight. For this purpose erect a perpendicular 

 pointer on the target and slide it along on the rod to the height of 

 the eye. The rodman should aim the pointer at the instrument 

 when the sight is being taken. If the rod is held vertically, iliese 

 tables will produce errors of 1 % at 8, 2'j at 11, 5% at 14 etc. 



* /> 



111 



