868

THEORY AND PRACTICE OF ENGINEERING.

Book 11.

Fig. 1110.

To trace on the Ground a Plan drawn on Paper -Having

drawn on paper the plan to be laid down, the angles ofwhich are indicated by the letters B C 1), E F G, II I K :drive a stump into the ground at the point where it is in-tended to have one angle of the plan, as at L, wherea cord L M must be stretched, towards the place in-tended to trace the plan; on the line set off L M, 52feet from L to M, answering for the 52 feet from B to C;and at this point N, drive a large stump into the ground;above the large stump or piquet, place a demicircle with itsdiameter in the line LN, for the purpose of forming theangle LNC, 123°, equal to BCD on the paper, setting offalso 36 feet from N to P, the length of CD: drive another stump into the groundat P, and stretch a cord horizontally between the two piquets P and N ; drive other stumpsbetween the two, and mark the two lines formingthe angles on the head of each stump: at thepoint P draw a re-entering angle N P Q, 105°,to equalise CDE, and set off 86 feet on PQ,from P to R, answering to D E on the plan; andcontinue in the same manner to form the angles,

RFN, RST, STL, &c., according to the plan,with the lengths of their sides: when thereare re-entering angles on the plan, take careto observe if the sides of the plan on theground agree with those on the paper, which maybe rectified by means of fixed points; for ex-ample, if the side It S was prolonged, it wouldexactly fall on the side TV, at the distance of15 feet from V in L, as is marked from II to X,on the side H G of the plan; if it falls near T,

some of the sides or angles are too small, if near V the contrary is the case.

Drawing and measuring Angles on the Ground by a divided Port Crayon

and two Cords _The divided port crayon may be of any length, from

5 to 6 inches, and ] of an inch square; the longest are the most conve-nient, on account of the two equal and parallel lines engraved on them :the line BC must be divided into 180 equal parts, according to the lineof chords on a sector; and D E, which is parallel to it, and of thesame length, should be divided into 60 equal parts: the two cords may bemade of fine twine about 36 feet long, as F, G,H, which must be dividedinto two at G, and a ring attached there ; and also its two extremities, Fand II, made large enough to receive a small stump ; divide F G and G Heach into 30 equal parts to serve as feet, &c. : the second cord F I,which is longer than the first, must

Fig. 1111.

; j ii'

•-’in

be divided into 60 parts, equal tothose on the other cord. Todraw an angle on the ground,as one of 40°, at fig. 1115, planta stump at the point where theangle is to be drawn, pass thering G of the cord oyer thestump, stretching the side F Gtowards the place it is requiredto fix one side of the angle, anddrive a stump through the ringF; then remark the port crayonon the line of chords B C, where40° coincides with the line DE,divide into 60 equal parts; havingobserved that it is at the 21st,stretch the cord FI, until its 21stpoint touches the ring II, where astump must be driven; so that on taking up the cord the stump F G H remaining, thethree points of the required angle are given.

One cord may be considered as passing the whole length of the spiral figure, its first ringat H, its second at G, and a third at the extremity, F, comprising 60 feet: the other cordcontaining 30 feet, or half the former quantity, has one ring at H and the other at G ; bythe application of these two cords angles may be set out on the ground: sometimes, instead

Fig. 1112.

Nil*

Fig. 1113.