45 



Thus, the telescope being leveled, the gradienter screw was turned through a 

 space of 11'"- 23 d "' required the arc : 



11 revolutions, = 3 9' 10" 



20 divisions, =0 6 50 



3 =010 



The whole arc, .*. . =3 17' 00" 



Conversely, it was desired to turn off a vertical angle of 4 35" 

 Then we have 



The space on the head of the screw = 16 r * T - 2*'- .0 



The engineer will bear in mind that the examples given are purposely given in 

 detail : that in practice the operations may be mental ones. 



It will be seen that the vertical gradienter can be used for a variety of purposes ; 

 measuring distances, grades, differences of levels, vertical angles, and is a useful 

 check against errors of rod or chain measurement. 



Messrs. C. L. Berger & Sons have also applied the same principle to their hori- 

 zontal tangent screws. By graduating a silver head attached to these screws sub- 

 divisions of one minute of arc are readily made. 



For constant use with these screws it is better to have a rod with two movabh 

 targets, or a rod painted with white and black squares as used in the coast surrey. 



Stadia Lines 



The gradienter screw is so universal in its application and can be so readily used 

 for angular, distance or grade measures, that it will generally be found best to have 

 it upon transits designed for current work. There are some cases however where 

 stadia lines are more expeditious in use than the gradienter screw, and give quite 

 as exact results. 



Stadia lines, for instance, where an instrument is to be used for distance measures 

 alone, commend themselves for their greater simplicity. For such work, non-adjust- 

 able lines, in connection with an inverting eye-piece, give the best results. If the 

 lines are adjustable, in the field usage of an instrument they may alter their distance 

 apart ; and there is a rapidity of work with fixed lines, and a rod graduated for 

 telemetrical work, which is not readied in any other way. 



These lines may be webs, or platinum, or they may be ruled on glass. The lat- 

 ter are extremely accurate, but the use of them is necessarily limited in the tele- 

 scopes of field instruments for the following reasons : thin as the glass may be on 

 which the lines are ruled, and intercepting only a small amount of light, yet the film 

 of dampness and dirt soon collecting on it will intercept a great amount of light 

 which in time may become a very serious impediment in the use of the telescope. 

 Another objection to their general adoption consists hi the fact that as the image 

 of an object is focussed in the plane of these glass-lines, a portion of the light of the 

 image will become reflected from the polished surfaces of this glass, causing at 

 tunes a disturbance in the clearness of vision. Besides, this glass-" micrometer," 

 as placed in most telescopes, is very difficult of access and must needs be removed 

 for cleaning, thereby increasing the liability of becoming broken, or detached from 

 its mounting. 



Section 

 showing the 

 Diaphragm 

 natural Size. 



