ao Nov., 1910.] Irrigation. 687 



Better than all this, is a small scientific instrument about the size of 

 ..a large watch called an "optical square,"" and costing aljout 12s. 6d. 

 It will save many a land-owner a lot of trouble in settmg out lines for 

 his buildings, yards, fences, &c.. and. with the chain, will enable him to 

 •do mostly all the surveying he is likely to require. The instrument 

 comprises a set of mirrors so arranged that, on looking through the 

 apertures provided for the purpose, square lines are at once apparent, 

 and all that is necessarv is to fix a peg where the instrument indicates, 

 and proceed as before. 



For example, let it be recjuired to set out a square line, B x, from the 

 point B in the fenced line A B C. Stand at the point B and, with the 

 right hand holding the instrument to the eye in a horizontal position, gaze 

 .as nearly as mav be estimated in the direction of x. Slightly move 

 the instrument horizontally with a circular motion until it reflects the 

 fence A B, and makes it appear to be on the line B x. Though not 

 -absolutely essential, it is well to send an assistant a few chains along 

 the projected line, and move him back and forth until the instrument 

 .shows that he coincides with it. \Vhen that occurs, he should drive in 

 .a peg, and if the instrument is in order it will indicate the desired line. 

 If the instrument is out of order, the point x will be made to appear at 

 y. To correct this, fix a temporary peg there; turn the instrument upside 

 down and proceed as before, and it will reflect the fence B C along the 

 line B z. Fix another temporary peg at z. Half the distance between 

 these two pegs will give the point x which will Ije at light angles to A B. 

 The instrument should l)e tested from time to time in this way to ascer- 

 tain if it is correct. Should it at any time be badly in error, take it 

 to the shire engineer or send it to the instrument maker. 



Set up the level at or about the point x shown on the plan, and having 

 •carefully levelled it proceed to read the staff at each peg, having pre- 

 viously ascertained, either by ohservation or otherwise, that the lanrl to 

 be graded is below the level of available water supply. 



The Tevel. — A description of the level, as a preliminary to explaining 

 its uses, will be of .some assistance, though, as in most other cases, an 



-ounce of practice is worth a ton of theory. In any case it will serve 

 as a reminder when doubts arise due to the .somewhat long intervals that 

 may occur between successive usings. 



Most levels, in addition to the level placed longitudinally above the 



'telescope, are furnished with a short transverse le\-el near the object end 

 of the instrument. Its purpose is to serve as an assistance in .setting the 



"instrument up, after which it is not referred to. Having screwed or 



-clamped it upon its stand or tripod, it will be found a good plan when 



netting up to firmly plant two legs of the tripod first. Then, with the 

 telescope pointing over the remaining leg, move that leg in or out, and 

 right or left, until the bubbles of both levels are near the centre of their 

 run, taking care to keep the point firmlv pres.sed into the ground. Then 

 proceed to centre them by means of the levelling screws. Some levels 

 are provided wdth three and others with four levelling screw\s. and for 



■simplicity of working and booking alone the former is to be preferred. 

 In the four-screw instrument bring the telescope over two opposite screws, 

 having previously slackened all four screws to prevent jambing. Revolve 

 these screws in opposite directions until the bubf)le comes to its centre 

 and the screws are felt to slightly grip. Bring the tele.scope over the 



•■opposite pair of screws and repeat. Continuing to move the telescope in 



