' 



sn;. 



Sl'liVKYIM!. 



010 



Mirvey may l>e completd by actual ad 



;, 



rit.s with tin- chain, in 



ii, nnd proceeding across the ground, measure the whole length of any 



I'olm having been set up at the angular points of every triangle, the li 

 imrveyor and hU onutant net out from any one of theo ]H>inU, as ' at 



one line, aa BC, leaving pickets in the ground in the direction of the 



line at or nea < where it crosses a hedge, as at , a - 



aa at 6, a road, or any other object which is to bo introduced in the 



plan. Should it happen that the line B c, or any part of it, coincide 

 with the direction of a road, offsets, as they are called, are me, 

 jwrpendicularly to the line, on one or both sides of it, in or.li i to 

 express the distance from the line to the sides of the road, or to the 

 hedge* or walls along those sides ; these offsets (which are shown at r, 

 rf, and e) should be measured at the end of every chain's length, and 

 particularly at every point in the station line opposite to which there 

 is a remarkable object, as a house, a gate, or merely a bend in the 

 direction of the road. Thus, when the work is laid down on paper, the 

 precise form and breadth of the road will be expressed. 



In like manner the other side, A c, of the triangle, is to be measured 

 with the chain ; pickets are to be left in the ground at or near every 

 place where a stream, as at h, a road, or a hedge crosses the side of the 

 triangle ; and offsets, as at k, I, m, are to be measured from the station 

 line to and acrowi such boundaries as may be nearly parallel to any 

 part of its direction. The like process is to be followed on each side 

 of every triangle ; the measured lengths of the sides of each triangle 

 should then be compared with the computed lengths; and if the 

 difference be not considerable, the work may be considered as having 

 l>erformed with sufficient accuracy; otherwise the operations 

 must be repeated, in order that the source of the error may be 

 detected. 



To carry on the work in the interior of any angle, as A n F, the 

 surveyor, where it is possible, measures with the chain the direct dis- 

 tance from the pickets in one side of the triangle to the pickets in 

 another ride, as HP, pq ; and since these pickets are supposed to have 



placed near the intersections of boundary-lines (roads, M 

 or hedges) with the sides of the triangle, the lines last measured will, 

 at leant in part of their length, coincide with or be parallel to some of 

 the boundaries in the interior of the triangle; and the precise figures 

 of such boundaries will be determined as before by offsets from the 

 measured line to all the principal bends. The length of each of these 

 ary station lines maybe obtained by trigonometry, since tin- line 

 is the base of a secondary triangle of which the two sides an- known, 

 I -eing measured parts of two sides of the princii.nl triangle, and the 

 included l.etwecn those sides has been found by the theodolite; 

 ilterefon- the measured length of thin line, on being compared with 

 the computed length, will afford an additional test of the accuracy of 

 the work. 



In measuring these secondary lines within each principal triangle, 

 pickets must, as before, be left in the ground in the direction of the 

 line, at or near places, as at r, where hedges, walls, Ac., cross the line ; 

 and from one of these pickets, r, to another, as t, lines are aft. 

 to be measured (these being M much as possible in or near the din < 

 i-tliiT boundaries), till at length the whole Ulterior of each 

 principal triangle will have been divided into several scr 

 triangles, all the sides of which have been measur. 

 by means of the offsets which have been measured from thi'iu, deter- 

 mine the figures of all the natural and artificial boundaries within the 

 tract of ground. 



Tin- M> ii.it ions of the buildings are also determined by offset- 

 the rtation lines nearest to them: the ground plans of them., 

 siderable edifice*, M churches and mansions, are measured, and the 



directions of their fronts with respect to the meridian are ascei 

 by a compass or otherwise. 



When rivers or roads have many abrupt and deep bends, the 

 initiation of their forms by means of offsets from the station-lines may 

 become impracticable ; and then a separate survey of such detail - 

 be made by means of the compass, the circumferentor, or the theodolite, 

 [THEODOMTK.] The same means must be employed for the survey of 

 a sea-coast, when the operation is to be performed on land ; and it may 

 often be advantageous to determine in like manner the forms ..f the 

 hedges, walls, &c., in the interior of the tract which is to be sui 

 Ground covered with wood must be surveyed by going quite round it ; 

 poles being set up at remarkable bends on the contour, the di^ 

 between them are measured with the chain, and the bearings ot the 

 several lines from the meridian are observed with the then.'' 



In order to explain the process of surveying with the theodolite by 

 the method which is commonly called that of the " back-angle," and 

 which is now almost constantly adopted, let it be required to determine 

 the outline it K CL B A, which may represent the contour ol 

 of \\liieh part may coincide with the course of a road or river. 

 instrument may be set up at H, which may be supposed to be tl 

 station: and let the line ns at each of the station* n, K. r, Ac . 

 sent the position of the needle or of the maiiiictic meridian at the 

 station : also let the instrument be adjusted so that the zero point of 

 the horizontal limb may be under the point n (the north point of 

 the needle), or the zero of the degrees in the compass-box n 

 in coincidence with ; and let K be the second station. Turn the 

 upper horizontal plate with the telescope till the object-glass of the 

 latter is directed to K, and make the intersection of the -.vires 

 to coincide with the object at that station ; then the index of the 

 vernier will be at some graduation " 'h p lower horizontal pi 



ml the angle n a K is that which is observed; siippo.-e it to lv 

 54", reckoning from the north towards the east, which angle is usually 



ited by N. 54 K. [N.B. Previously to directing the < 

 glass to K, it might have been directed tn any other visible oi 

 as p or D, whose positions it mi ;ht be required to determine by means 

 of their bearings from the meridian line.] 



Let the theodolite be now removed to K, a staff being planted in 

 the ground at n : turn the whole instrument round on its v. 

 axis Tthe index of the vernier remaining at the graduation N. .' 

 till tne object-glass of the telescope is directed to n, and the inter 

 section of the wires appears to coincide with the staff there. Then, if 

 Hi.' former angle were correctly taken, and no movement, of Hi 

 zontal plates on one another have taken place, the smith point of the 

 needle will lie over the zero of the graduations on the lower ].' 

 will coincide with the zero of the degrees in the c 

 and this circumstance will be a proof of the accuracy of tlie 

 all the meridian lines ?!,, being supposed to be po> 

 another. 



Now turn the upper horizontal plate with the telescope, till UV 



object-glass of the latter is directed to c, and the intei>eetiim of the 



]>].enrn to coincide with the object there: the teleseoj"' in 



moving from the position K H to the position K r bavin;' i 



and beyond * ; and the index of the vernier being supposed to be at ;/, 



