HORIZONTAL SECTIONS. 69 



surveyed (Part I.), commencing at a milestone in the 

 S.E. corner (fig. 8). On this the "broad-arrow" has 

 been cut and it is shewn thereby to be an Ordnance 

 Bench-mark, the height of which, above the sea, is 

 ascertained to be 365 feet. We may take the distances, 

 where our line crosses roads, brooks, and so on, from 

 the map, reserving the example of measuring by chain 

 for the theodolite illustration. But these points are too 

 far apart and at too great a difference of level for the 

 instrument to be placed once only between each two, 

 we must therefore take intermediate observations. 



Note. In running a set of levels by this instru- 

 ment and scaling the distances of definite points 

 from the map (the intervening surface line being 

 sketched in by eye and hand), it is immaterial 

 whether the intermediate levels be taken along the 

 line or by a more convenient route, so that each of 

 these points be made a station for an observation. 

 The Bench-mark, that is, the milestone of which the 

 reduced level is known, must be the starting-point, and 

 having drawn on the map our proposed line therefrom 

 and passing by the church, we proceed in the following 

 manner. The instrument is set up on the road a few 

 chains from the B. M., and the plate-screws turned until 

 the spirit- tube indicates that its position is in every 

 direction horizontal. The assistant holds up the staff 

 upon the milestone, so that its base coincides with the 

 upper line of the " broad-arrow " and a reading is then 

 taken therefrom. This must be entered in the level- 

 book as a " back-sight" (10*90), and a second look taken 

 through the telescope for the purpose of checking the 

 accuracy of the first reading. The man then takes the 



