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 directions. Attached to the telescope is a delicate spirit level and often a ring compass. The telescope is adjusted 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 950 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 necessary 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 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 intersect 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 figures recorded respectively are 2 ft. 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: -

Dumpy Level.

Fig. 403. - Dumpy Level.

Levelling Rod.

Fig. 404. - Levelling Rod.

Diagram showing how Flying Levels are taken between various points.

Fig. 405. - Diagram showing how Flying Levels are taken between various points.

Stations.

Distance.

Back Sights.

Fore Sights.

Rise.

Fall.

Reduced Level.

A

ft

ft.

in.

ft.

in.

ft.

in.

ft.

in.

ft.

in.

-

-

-

-

-

-

B

50

3

0

5

0

-

2

0

2

0

C

,,

2

0

5

6

-

3

6

5

6

D

,,

3

6

3

6

-

-

-

E

,,

4

6

3

0

1

6

-

4

0

F

,,

4

0

4

6

-

0

6

4

6

G

,,

2

6

5

6

-

3

0

7

6

19

6

27

0

1

6

9

0

19

6

1

6

7

6

7

6

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.

In the diagram it will be noticed that the dotted line hl represents the level, and it may be seen that there is just enough ground above this line to fill up the hollows beneath it and thus make the surface perfectly level. If it is desired to lay out ground on an inclined plane, instead of perfectly level, a rod is placed at each end, and one in the middle, as shown at A, G, and D, respectively. The line ip will represent the inclined-plane surface, and the point at D being in the middle will remain in the same position on the surface. From the diagram it is obvious that it would not be necessary to move as much soil to fill up the hollows below the line ip as it would to fill up beneath hl. The levels between A D and D G should be taken in the way already described, making A and D extremities in one case, and D G extremities in the other. In this way a good deal of labour in shifting soil afterwards would be saved, as the hollow places would be indicated by the section in the field book.

In connection with ordinary land surveying for horticultural and agricultural purposes one need not consider such problems as the curvature of the Earth's surface, nor is there any great need for the use of the theodolite and trigonometry, or logarithmic tables.