870

THEORY AND PRACTICE OF ENGINEERING.

Rook 11

Fig. 1118.

Fig. 1119.

To draw by means of a Scale andProtractor a Plan , greater, equal , orsmaller than a given Plan. —It isrequired to copy the plan A, whichis bounded by six sides, B C, 1) E,

&e.; the scale II being divided into100 parts of the same size. Drawa scale K of the same length as II,viz. into 100 parts; measure howmany parts of this are contained bythe side B C, as 80 ; draw the lineL N, on which set off 80 parts, astaken from the scale from L to N :then with the protractor measurethe angle CBGof 117°, and drawanother equal to it at the pointN, having its diameter along L N,and count 117 degrees on its circum-ference, beginning at L: having removed the protractor,draw the right line N P, off 60 equal parts, to answer tothe side B G: at the point P draw the angle NPQequal to B G F on the plan A, and set oft' as manyparts on P Q as are equal to G F on the plan A: con-tinue to draw the relative sides and angles of theplan A, until you have the plan L N P Q It S equalto it.

Copying Plans by means of Squares. — To copy theplan A, it may be inclosed in squares, as HIKL-divide the two opposite sides HI and KL into thesame number of parts, as into five, and draw rightlines through the opposite points of these two lines:divide also the sides IIL and IK into any num-ber of parts, and draw lines through the oppositepoints, which crossing the first will form several squares.

To copy the plan A precisely the same size, drawthe squares equal, and then tracing the map or planthrough all the corresponding parts, they will be alike.

If it be required to copy a plan without drawing,squares on the originals or on the copy, fine threadsstretched across may be made to answer the same pur-pose, and on the paper on which the drawing is to bemade, fill up the squares in the manner of the original:thus the copy may be completed without ruling anylines on the original.

The Cross Staff" is commonly of copper or brass; there are both simple and com-pound : the simple one, A, is a circle of brass, 4 or 5 inches in diameter, dividedby two lines at right angles, which have their extremities mounted with sights; thespace between the arms is hollow, to render it lighter, and more portable; below thecross is fixed a joint with a dowel to fit into a socket joint, fixed on the staff* whichsupports it.

The cross staff*, F, is compound, 6 or 8 inches in diameter; it has four sights, andan alhidade is fitted to its centre by a two-headed screw, as K L, whose extremities

Fig. 1120.

Fig. 1121.

Fig. 1122.

I'ig. 1123.

Fig. 1124.