840

THEORY AND PRACTICE OF ENGINEERING.

Rook II.

taking angles of elevation or of depression as by the theodolite or quadrant,, which latteris divided into degrees, and furnished with a plummet suspended from the centre.

To draw the Map of a Country with the SquarePlane Table. — Choose two stations, as A and B,and plant two piquets where the whole ofthe country it is intended to map may beseen. Then, having measured the base or stationline A B, draw on the plane table the line L K,near one of its sides, and placing the plane tablemounted on its feet at the station A, turn it insuch a manner that the point K of the stationline shall be above the point A, with the planetable turned towards the places to be taken,and in such a manner that we discover the pi-quet B in the visual ray of the two piquets Kand L Place the rule against the pin K, andturn it, boning it towards the places to be taken,and draw the lines K, C, D, E, I; remove the tablefrom A to B, so that this point D may be abovethe piquet B, and that we can discover the pi-quet A in the same visual ray with the pinsLK; so that by placing the rule against the pinL we can bone the places, observing the linesI,, C, D, E, F, in order to note where the lines drawnfrom the second station cut those from the first; these 1 'points will give the just position of the places inthe country required to be mapped. But if it be re-quired to find how far these places are from each other,it will only be necessary to measure the distance be-tween the stations A and B, and to divide the station

line into the same number of equal parts, and then it ^

will serve for a scale for the whole. If from the two Fig. 1065.stations A and B we cannot see all the other places,

F, G, II, then one or more other stations will be necessary.

To draw the Map of a Country with the Round Plane Table , which is furnished with cards,each of which has a radius or semi-diameter marked upon it, to answer and serve as stationlines, and mounting its stand in some elevated ground, piquets are planted to show thestation points, and the distance being measured between them, as in the line A B, 742 feet,serves as a scale. Place the plane table at the piquet A in such a manner that, whenboning along its rule or telescope, we can see the piquet B. Then draw along the rulea station line, on which write station from A to B ; which being for all the objects whoseposition we would have, take the card from the table, by turning the screw which holdsthe alhidade or rule, and place a fresh card on the plane table; then go to the piquet B.placing the centre of the plane table above it, and turn it until we discover by the side ofthe rule the station A, and draw the station line, writing on it as before, station from B toA ; bone from this station B all the objects boned from A. and draw all the lines tothe visual rays, writing the name of the object on them as before. Having marked on thecards all the places of the map intended to be laid down, draw about the middle of a sheetof paper the line C D, to serve as a station line, remarking that the map will be largeor small in proportion to it; place the centre of the card drawn at the station A onthe point C, and make the radius of the card on which is written station from A toB coincide with the line Cl) ; prolong it to infinity on the map to be made by means ofrules; draw all the lines marked on the card, and likewise place its centre drawn at thestation D over the point D, making its radius coincide with the line C 1) ; prolong onthe paper all the lines marked on the card B and the points of intersection, which thelines prolonged from the centre B make with those from the centre A, which will showthe positions on the required map. To form a scale, set off the length C D, and divideit into as many feet as were measured from A to B.

The Theodolite has been brought to such perfection, and is so complete an instrument forthe taking of large or small surveys, that it has almost superseded all others; formerly theyconsisted of a whole or half circle, about 10 inches in diameter, divided into 360 degreesbut angles that were vertical and horizontal could not be taken by it at the same time.

'The theodolite now in use consists of two circular brass plates, which turn one upon theother, and have a horizontal action by means of an upright axis, which is made of twoparts, external and internal; the former is secured to the lower, the latter to the upperplate. The lower plate has its circumference divided into 360 degrees, which arc againsubdivided, and at the extremities of a diameter of the upper plate are fixed two

Fig. 1064.