be used for observing or establishing gradients, for determining 

 distances with either a fixed or variable rod interval, or for measur- 

 ing small angles without the use of the graduated limb. 



Theory 



The principle of its construction is very simple. If the radius 

 of the clamp \\ hich controls the movement of a sij;ht line is divided 

 into 100 parts and this value used as the pitch of the thread for the 

 tangent screw, it is evident that one revolution of the screw will 

 communicate a movement in the sight line equal to an angle of 1 % 

 or 34' 23''. In other words, the tangent of this angle is equal to 

 1 ft. at 100 ft. from the center of the instrument, 2 ft. at 200, etc.* 

 The length of clamp which we use would make such a pitch too coarse 

 for ordinary requirements so that, by preference, we take l-200th 

 of the clamp radius and turn the screw two revolutions for each 1 % 

 to accomplish the purpose. 



If the tangent screw of the transit telescope is provided with a 

 drum head that is divided into 50 parts, each division will then 

 represent .01$ or will encompass .01 of any unit of measurement 

 on a rod at 100 such units from the center of the instrument. 

 Just above the graduated 

 drum is an indicator scale 



of spaces equal to the pitch U\ 



of the thread, merely to 

 measure the number of 

 whole revolutions, the 

 fractional part being taken 

 directly from the drum. 



Pi*. i< 



Encinetrins and Surging Instrument*. /. O. Baker, 1906. p. 209. 

 121 



