HORIZONTAL SECTIONS. 77 



the assistant himself, as on the top of his head or the 

 line of his eyes. 



But the fore- and back-sights at an object must always 

 be taken on it when at the same level ; if this be done it 

 does not matter about its being a little too high or a 

 little too low relatively to the surface. For instance, it 

 may sometimes be convenient to place the man, with or 

 without the staff, on the top of a bank or on a gate 

 this will cause a local error of a few feet, but if both 

 fore- and back-sights be taken on it without alteration in 

 its position a constant error is avoided, which would 

 otherwise be carried on to the end of the section. 



If the line of section, across the map (figs. 5 and 8), 

 were to be run by theodolite, the proceedings would be 

 somewhat as given below. Assuming that the start be 

 made from the milestone, which is also an ordnance 

 B. M. 365 feet, the first thing to be done is to find the 

 line (N. 35 W.) along which the section has to be run. 

 The instrument is set up and manipulated by the plate- 

 screws until the spirit-levels indicate that it is in a 

 horizontal position, then turned round, with the needle 

 free, until the latter comes to rest at 13 (35 W. 22 

 variation =13), the final adjustment having been made 

 by the tangent screws. The telescope being undamped 

 is free to move vertically (only), and as it now points 

 directly along the line of section, any objects in the 

 centre of its field are also in the line and may be pointed 

 out to the chain-man for his guidance. We may assume 

 that a tree in this instance comes in, and beyond that 

 the line passes just between two cottages seen at a long 

 distance. After seeing that the leading man has his ten 

 arrows, and that the chaining has been started in the 



