LAND SURVEYING. 571 



the line D B, 750 links, distinguished into D a 150, a b 270, b c 

 150, and c B 180; and the perpendicular offsets a d 150, 6 e 90, 

 and cf 180. 



Offsets are usually measured, unless they be of great length, 

 with the cross-staff, which is made 10 links long for this purpose. 

 If, however, these perpendiculars exceed two or three lengths of 

 the staff, it is better to use the chain. These offsets, it is also to 

 be noticed, are to be measured in the course of ascertaining the 

 line D B. 



In simple surveys, such as the preceding examples refer to, it 

 is usual to sketch the form of the field on a page of the book, and 

 enter all the measurements and remarks on the corresponding 

 parts of the sketch ; this is an excellent plan, and should always 

 be adopted where the case will admit of it. 



II. We come now to the plotting, or laying down the true 

 figure of the field on paper, according to some determinate scale ; 

 and this operation is no less simple than that of taking the field 

 measurements. The surveyor requires to be provided with a case 

 of drawing instruments, containing compasses, steel pen, parallel 

 ruler, &c. ; and also a plotting scale, graduated into chains, arid 

 parts of a chain. The paper on which the plotting is to be made, 

 should be damped on the back, and fastened down by pasting the 

 edges on a drawing-board of suitable dimensions. When dry, the 

 paper thus prepared presents a smooth and uniform surface ; 

 whilst the board on which it is laid prevents the points of the 

 compasses from piercing deeply into it and making unsightly 

 holes. 



We will begin by plotting the field ABC, the measurement 

 of which forms the first example in the preceding section. 



Draw an indefinite line and set off from the plotting scale the 

 base A B = 900 links, marking with the point of a pencil the 

 terminations A and B ; set off on A B the distance A F = 300 

 links ; from the point F, raise an indefinite perpendicular, and 

 set off on it F C = 480 links ; draw lines connecting the points 

 B C, A C, and the field is correctly plotted. The field A B C D, 

 it will be obvious merely requires the erection of the perpendicu- 

 lar D E = 510 links, and drawing lines connecting the points 

 A D, D B, in addition to the foregoing operation. 



In the third example, where the boundary D d ef B, is irregu- 

 lar ; on the line D B set off D a = 150 links, a b = 270 links, 

 b c = 150 links, and c B = 180 links ; and from the points a, b, c, 

 raise the perpendiculars a d = 150 links, b e = 90 links, and 

 cf =180 links. Draw lines connecting the points D d, d e, ef, 

 f B, which finishes the plotting of the whole field A C B/e d D. 



It is usual to place the north side of the plan uppermost, with 

 a flower-de-luce to indicate the north point, and in a vacant 

 place to insert a scak of equal parts or chains, and the title of 

 the plan. 



III. The next thing to be done is the computing the contents 



