Garden Surveying, Levelling, and Mensuration 219 



sightings are taken from the dumpy level midway between the stations. 

 Thus by sighting on to A the hair line intersects the rod at 3 ft. from the 

 ground. This figure is put in the 

 back-sight column. Then, looking 

 on to shaft B, it is found to inter- 

 sect at 5 ft. from the ground. This 

 figure is entered in the fore-sight 

 column. The instrument is then 

 moved midway between stations B 

 and C, and B now becomes a back 

 sight, and C the fore sight. The 



. J Fig. 405. Diagram showing how Flying Levels are 



figures recorded respectively are 2 ft. taken between various points 



and 5 ft. 6 in. In the same way the 



instrument is moved midway between the other stations, and the figures for 



back sights and fore sights are recorded as shown in the following table: 



The back sight to any station is usually placed on the line below the 

 point to which it refers, hence the back sight for A 3 ft. is placed on the 

 line opposite station B. When a back sight is greater than a fore sight, 

 the difference is placed in the "rise" column; but when less, the difference 

 is placed in the "fall" column. The difference between the sum of the 

 back sight and the sum of the fore sight gives the "reduced level". Thus 

 in the above the reduced level for B is 2 ft. and at C 5 ft. 6 in., because 

 there has been a further fall of 3 ft. 6 in., which must now be added. At D 

 there is nothing to add or subtract, the back sights and fore sights being 

 equal. At E there is a rise of 1 ft. 6 in., and this must be deducted from 

 the previous reduced level of 5 ft. 6 in., thus leaving it at 4 ft. The 

 further falls of 6 in. and 3 ft. must be added, making the reduced level 

 for the whole operation come to 7 ft. 6 in. It will be observed that this 

 is exactly the difference between the total of the back-sight column and 

 the fore-sight column; as it also is the difference between the totals of 

 the "rise" and "fall" columns. If the difference is not identical in each 

 case there is something wrong. 



