PART II. FOREST MAPS 



SECTION I 

 THE TRANSIT 



THE transit in general engineering work is the most 

 useful and most frequently employed of surveying instru- 

 ments. It is commonly used to measure horizontal and 

 vertical angles, but, having a magnetic needle, it may be 

 used to take bearings, and, when provided with stadia 

 wires, to measure distances. It may also be used as a 

 levelling instrument. A cut of a transit is shown here- 

 with, also a sectional view through the axis of the same 

 instrument. 



The essential parts of an engineer's transit are described 

 below. The telescope is attached by means of a hori- 

 zontal axis and standards to the upper of two circular 

 plates. The two plates move freely on one another, the 

 lower being graduated, while the upper has a vernier 

 which allows readings to be made with accuracy. A 

 compass circle is also attached to the upper plate. A 

 clamp fixes the upper to the lower plate, and a tangent 

 screw secures a slow adjusting movement between the 

 two. A similar arrangement is placed between the lower 

 plate and the head of the instrument. 



The whole instrument is supported on a tripod ; levelling 

 screws serve with the aid of cross levels to fix the plates in 

 a horizontal position ; and a finely turned spindle and socket 

 arrangement guides the plates in their movement on one 

 another. By means of a plumb line attached to the lower 

 end of the spindle the instrument may be set with its axis 

 exactly over any desired point. 



1. ADJUSTMENTS OF THE TRANSIT 



The object of these adjustments is to cause (1) the 

 instrument to revolve in a horizontal plane; (2) the line 

 of sight to generate a vertical plane when the telescope is 

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