838

THEORY AND PRACTICE OF ENGINEERING.

Book II.

Fig. 1059.

be made by measuring one of the sides on the ground, .as E F, in feet, and dividing itscorresponding sides on the plane table into as many equal parts.

To take the Plan of a Place where entrance is denied , as that of the town A B C D E F :plant piquets or other marks which may be distinguished, and having measured one of thesides as E F, 236 feet, draw on theplane table a station line G H,which divide into 336 parts, andfix a pin at each extremity of it:place the plane table at the stationF, so as to be towards the angle orpoint G, on the station line pre-cisely. Place the rule againstthe pin G, and turn it until theangle B is seen ; then draw alongthe rule, the line GK; turn therule towards the point C, and drawthe line G L ; in the same way bonethe line D, and draw the line G M.

But for the angle E it is not ne-cessary to bone it, as we have italready laid down. Remove theplane table from F to E, and turnit in such a manner that the pointII corresponds rightly on the station line and the side FE, so that by putting the ruleagainst the pin II the angle A may be boned, and draw the line II N; in like mannercontinue to draw the other lines II O, II P, II Q, to the angles B, C and 1), in order to remarkwhere these lines cut those of the first station, on the points R, S, T, V, that by drawingthrough those points of intersection right lines, we may form with the station line GHa plan R S T V H G similar in all respects with that of the town A B C D E F.

To draw from a given Point a Line parallel to an inaccessible Length , as that of the lineB C : plant a piquet at D, the point through which the line is to pass, and another as at E:measure the distance between these two piquets as

50 feet; divide the station line F G on the plane -

table into 50 parts, to answer to the 50 feet from — ■■■

D to E.

Place the plane table mounted on its foot insuch a manner that the point G on the stationline shall be above the piquet E, so that whenboning by the two pins on the station line G F,the piquet D may be seen in the same visualray ; place the rule against the pin G, in orderto bone the two points B and C, which will enablethe lines G H and G I to be drawn on the plane table. Then remove the plane tablefrom the station E and place it at I), disposing it in such a manner that when honing by thetwo pins F and G, we can discover the piquet E, and placing the rule against the pin F,the two points B and C.

Draw the lines FK and FL on the plane table, observing where they cut the lines GIIand G I, in the points M and N, in order to draw the line N, which will be parallel tothe inaccessible line B C.

Draw on the ground from the point D a line D O, parallel to this line AI N, and the lineD O will also be parallel to the line B C.

To measure the Height of accessible Places , as that of the line B C. Plant a piquet D atany distance from the foot of the tower or line B C, as at 40 feet. Divide its station lineinto 40 parts corresponding with the 40 feet from Bto D: dispose then the plane of the table in such amanner as to he quite perpendicular with the horizon,and its station line parallel to it, and the point F onits station line over the piquet D. Place the ruleabove the pin F, lowering or raising it until wediscover by the side which touches the pin the pointC ; then draw along the rule on the same side as thepin the line FII ; from the point E, on the stationline, elevate the perpendicular E I, and remark wherethis perpendicular cuts the line FII on the planetable, as at K: the height E K measured on thestation line K F will give a height, to which addthat from the point F on the station line to the sta-tion point I), and we will have the precise height B C.

Fig. 1CG0.

Fig.1061.