2l8 



Commercial Gardening 



is possible to see whether the third rod is higher or lower than the second 

 one. This latter must be arranged on the same level as the sighting one 

 by means of a spirit level, after which the third rod may be shifted about 

 to various spots. Pegs are driven in at the various points until the tops 

 are at the required level. One can then see at a glance, by the length of 

 the pegs sticking out of the ground, which are the higher and lower places. 

 The diagram (fig. 402), shows another set of boning rods. The sighting 

 hole is in a plain upright rod without a cross piece, while the 

 end rod has a rounded cross piece, with the portion above the 

 proper level painted white. This makes it much easier for the 

 eye to distinguish the edge of the second or intermediate rod 

 when sighting through from the first on to the third. 



The Dumpy Level. 

 For more accurate work 

 an instrument called the 

 "dumpy level" (fig. 403) 

 is useful. It consists of 

 a telescope mounted on a 

 tripod stand, and can be 

 moved about in all direc- 

 tions. Attached to the tele- 

 scope is a delicate spirit 

 level and often a ring com- 

 pass. The telescope is ad- 

 justed properly when the 

 bubble in the spirit level is 

 exactly in the centre when the telescope is turned backwards and forwards, 

 and when the cross hairs on the lens are distinctly seen. The attached 

 level is then said to be parallel to the " line of collimation" of the telescope. 

 In using the Dumpy level the tripod is fixed firmly on the ground and 

 the operator looks through the telescope on to the levelling rod placed on 

 a certain spot. The levelling rod (fig. 404), varies in height up to 14 and 

 18 ft., and has three pieces which may be drawn out if necessary. It is 

 divided into feet and decimals of a foot as shown. The feet are shown 

 in red figures, the odd numbers of the foot decimals in black figures, while 

 the hundreds are shown by black and white lines alternately. 



In simple levelling the dumpy level remains in one position, from which 

 all readings are taken. The first reading is called the " back sight " and 

 the last the " fore sight". The difference in the readings of the back sight 

 and fore sight is equal to the difference of level of the two points. Thus, 

 if the back sight is read as 9'50 ft. and the fore sight at 13'75 ft. there 

 is a fall from the back-sight point to the fore sight of 4-25 ft. 



Where, however, the ground is of an irregular nature it may be neces- 

 sary to take several levels. These should be entered up in a field book 

 divided into columns as shown below. 



In the diagram (fig. 405) the stations are marked from A to G, and the 



Fig. 403. Dumpy Level 



Fig. 404. 

 Levelling Rod 



