lo Dec, 1910.] 



Irrigation. 



767 



may be necessary, and it will be well to number the stakes consecutively,, 

 starting with o at the initial station. By thus identifying them it simpli- 

 fies references should it be desired to lay off laterals from the original 

 line. With the stakes all in position, it is a simple matter to average them 

 and produce a series of straight lines, shorter or longer according to the 

 natural slope of the country. An example is given on the opposite page. 

 With the plough mark out the straight lines shown, but making easy 

 curves or bends at each angle, or bend, and proceed to construct a head 

 ditch or channel as described later. 



Contours.— -Where water has to be carried some distance as in a 

 channel, "contours on grade" are marked out, i.e., contours having a 

 regular fall per mile. The only variation from the method above described 

 is to make the staff reading on each change peg (that is, the last peg 

 before moving the instrument) that proportion of the amount of fall per 

 mile determined upon, that the distance between back and fore sights 

 bears to one mile. For example, assuming the desired fall to be 6 inches 

 = .50 ft. per mile, and that the distance between back and fore sights 

 is 8 chains. If the staff-reading at original peg No. o is 4.50 ft., then 

 the peg at 8 chains, No. 8 (= i-icth of a mile), must be set so that the 

 staff reading is 4. 5 :; ft., or .05 ft. lower (= i-ioth of qo ft.). The inter- 

 mediate stakes Nos. i to 7 may all be set at 

 the original staff reading of 4.i;o ft.: in the 

 majority of cases nothing is gained by inter- 

 polating values for them. The process may of 

 course be continued to infinity, but to guard 

 against errors and to provide easy reference for 

 any extension of the system afterwards, or the 

 determination of foundations for works such as 

 bridges, culverts, &c., it is well to take note of 

 the levels of some fixed points at intervals 

 of 20 or 40 chains along the route. 

 Such fixed points are called Bench Marks ^ 

 (written B.M.) and are conveniently cut upon 

 existing trees, and in such a manner that the bark is removed from live 

 trees as shown by the shield in the drawing. The wood is cut into far 

 enough to admit of a small bench being left at the foot upon which the 

 staff is held. Letters are cut in the barked space as may be desired. 

 Where there are no trees fix permanent posts or pegs, and always in 

 such positions right or left of the route that they will not be 

 disturbed in the course of building the channel. These B.M.'s will have 

 a permanent R.L. (reduced level) and are easily recorded in the level book 

 for future reference. 



Boning Rods. — There is, however, another way by which a fairly 

 accurate knowledge of the surface may be ascertained where a land-holder 

 mav not have an instrument at command, but it rests on the pre-determina- 

 tion of some points with the level. The cost of establishing these points 

 would be very small compared to that of engaging a surveyor to make a 

 chessboard survey of the whole area. 1 refer to the use of " Boning 

 Rods," so familiar to contractors. 



A boning rod is easily made from a piece of fiooring board or any 

 other handy timber and will be best understood from the sketch. The 

 size may be varied to suit the needs of the operator. With a set of 3 of 

 these rods and a few points whose level is known it is a simple matter to 



BENCH .MARK ON TREE. 



