78 A MANUAL FOR NORTHERN WOODSMEN 



provided with two additional horizontal hairs, usually fas- 

 tened to the same diaphragm as the ordinary cross-hairs, 

 and placed at a known distance apart. The space be- 

 tween these two extra hairs is preferably fixed, but in 

 some transits the diaphragm is so arranged that it can be 

 adjusted. The instrument must also be provided with a 

 level on the telescope and a circle or arc for measuring 

 vertical angles, since the telescope is seldom level when 

 measurements are taken. 



Stadia rods are usually 10 or 12 feet long. They are 

 plainly painted in such a design as to be read at long dis- 

 tances. Engineers generally use rods graduated to feet 

 and tenths, the hairs cutting off one foot on the rod at a 

 distance of 100 feet. Hundredths of a foot are generally 

 estimated. For use in connection with a land survey it may 

 be more convenient to graduate the rod or adjust the hairs 

 so that one unit will be cut off at a distance of 66 feet or 

 one chain. 



Inclined Sights. The distance between instrument and 

 rod is measured directly if the sight is taken horizontally, 

 and a vertical angle between them of 5 or less does not so 

 affect the sight as to matter particularly in many kinds of 

 work. If, however, a sight of greater inclination is taken, 

 a reading is obtained that represents a greater distance 

 than the horizontal one between instrument and rod. If 

 for an inclined reading the rod is also inclined, so as to be 

 perpendicular to the line of sight, the reading represents 

 the inclined distance, and the horizontal distance is the 

 cosine of the angle of inclination multiplied by the inclined 

 distance. Similarly, the difference in elevation is the in- 

 clined distance multiplied by the sine of the angle. 



It is usual, however, and better, to hold the rod plumb, 

 and here the computation of horizontal and vertical ele- 

 ments is not so simple. Tables, however, have been com- 

 puted which give these elements, horizontal distance and 

 difference of elevation, directly. A compact stadia table 

 will be found on page 211 of this work and an example 

 showing the method of its use is given on page 80. 



What has been written above needs, however, one 

 qualification. Stadia wires to read truly at all distances 



