PREPARATION OF THE DESIGN 81 



Repeat the process for as long as required, not 

 omitting to reverse the straight edge at each level- 

 ling. That is to say if the board is represented by 

 A B, it is used alternately A B, B A, all along the 

 line, the object of this change being to counteract 

 any inaccuracy caused by warping or other injury. 

 Pegs over 2 ft. long are difficult to keep steady and 

 to obviate this at any point a peg may be driven 

 in alongside of the true level at a definite distance 

 below it, and the line continued. The foot or so 

 subtracted can be reckoned in at the end. When 

 the surface is undulating it is sometimes a good 

 plan to start with a peg a few inches above the 

 ground, to avoid having to excavate at any point. 

 The resulting line is so many inches above the 

 starting point and can be allowed for. Very good 

 practice in levelling can be had by working round 

 a piece of ground and seeing that the last peg 

 comes level with the first one. 



For large surveys a dumpy level is required. 

 This (without accessories) is a spirit level mounted 

 on a tripod and fitted with a telescope through 

 which a number on a staff can be read. Fine 

 wires are stretched across the glass and when the 

 spirit level of the instrument is true the horizontal 

 wire on the telescope shows the level on a staff held 

 perpendicularly by an assistant. It is not pro- 

 posed to enter into details about the construction 

 and setting up of the dumpy level, but only to 

 show its working, for if a survey is on a scale to 



G 



