836

THEORY AND PRACTICE OF ENGINEERING.

Book 11.

Fig. 1054.

placed on the plane table, folding down the edges, which are securedeither with mathematical pins or by paste ; sometimes the planetable is sunk, and by means of a frame the paper is kept flat.

When the paper is laid over the table T, the frame V passesover it, and keeps it perfectly stretched and in its place.

To take the Distance between two Objects , as A and B, which are eachaccessible from one to the other , the plane table is placed at any point,as C, opposite the middle of the two objects: draw a scale near oneof the long sides of the table, and fix a pin, D, at about an inchfrom this scale within the table, where the station line generally is;then turn the plane table in such a manner that one of its sidesshall be nearly parallel to the two objects A and B : place againstthe pin D the side of a rule which may be moved in such a mannerthat when boning along the face of this rule from the pin D, thevisual ray may cut the point A : draw on the paper from thepin D along the side of the rule the line D E : then shift the ruleto the other side of the pin, and in a similar manner draw the lineD F: measure how far it is from the station C tothe object A, as 64 feet, and take from the scale G 64parts, and set them off on the line D E from D to H,to answer to the 64 feet from the station C to the ob-ject A : likewise take 66 parts from the scale G, andset them off on the line D F from D to I, to answer tothe 66 feet from D to B : draw the line II I, whichbeing measured by the scale G, will give 51 parts forthe number of feet from A to B.

To measure the Distance between inaccessible Objects ,as that of A and B : place the two piquets, C and D,at pleasure, and draw on the plane table a scale, E,of any length or division, as into 600 equal parts, andmeasure the distance between the two stations C andD, as 563 feet: take from the scale 563 parts, whichwill give the station line F G on the table, on whichfix the pins F and G : place the plane table at one ofthe stations, as at C, in such a manner that the stationpoint F shall be above the piquet C, and the planetable towards the objects A and B; bone the piquetD by the two pins F and G, keeping the table in thisposition; place a rule against the pin F,and bone theobject A along the side which touches the pin anddraw’ the line F H. Then turn the rule against thepin, until the object B is seen, drawing in the planetable the line F I : remove the plane table from thissituation to the piquet D, placing it in such a mannerthat the point G of the station line shall be exactlyabove the piquet D, so that by boning along the pinsFand G the piquet C may be discovered: place therule against the pin G, and bone the object A alongit, drawing on the table the line from the pin G,along this rule a line GK, and remark where it cutsF H at L: turn the rule until the object B is dis-covered, keeping the rule against the pin G, and drawon the plane table along the rule a line G M, andobserve where the line GM cuts F I, as at N: thelength L N measured by the scale F will give 342 feet for the inaccessible distance A B.

In this figure two stations are selected as the extremities of the base line, the distancebetween which is accurately measured, and represented by a line drawn on paper, accordingto the assumed scale : if the modern plane table be used, it is set up at one of thesestations, and a fine needle or pin being stuck into the table at one extremity of the linedrawn on the paper, the edge of the index is brought to press gently on the pin, and co-incide with the line and the table turned round, till the object of the second station isbisected through the sight vanes ; the table is then clamped, and the direction of the mag-netic meridian marked ; the fiducial edge of the index, still kept in contact with the up-right pin which serves as a centre, is then directed successively to the objects A and B, andlines drawn on the paper in the direction of each : when this is done, the table is re-moved to the other station, and the pin placed at the correspondent point on the paper,which forms a second centre. The edge of the ruler is then directed to A and B as before,

Fig.1055.

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T'S C( IE

Fig. 1056.