834
THEORY AND PRACTICE OF ENGINEERING.
Book II.
Fig. 1049.
L, l>one the piquet E and the inaccessible objects A and B, as was done at the first station,which being observed at the station K, plant there a piquet for the second station. Thedistance between these two stations or piquets L, K, will be equal to that between the inac-cessible objects A, B; that is to say, if it be 180 feet from L to K, it will be 180 feet fromA to B.
To measure accessible as well as inaccessible Heights. — Place the cursor on the secondgrand division of the staff, holding the staff horizontal with the cursor perpendicular to thehorizon ; advance to or retire from the ob-ject whose height is required, until byboning along the staff and the two ex-tremities of the cursor the foot and thesummit are discovered at the same time,which when observed, plant a piquet atthe station of the same height as the staffis held. Then run the cursor on the thirdgreat division of the staff, and advance orretire from the piquet until at the sametime the foot and the summit of the objectare discovered, remembering to keep the staff'horizontal, as at the first station, and boneby the two extremities of the cursor; then
plant a piquet at this station: the distance between the two piquets will be that betweenthe objects.
The height of an object, A B, being required, fix the cursor at the second division of thestaff MN, as at H ; then holding the staff' horizontal, bone from its extremity M alongthe two extremities C and D of the cursor, until the foot A and the summit B are seen,which will be the case at L, where a piquet must be planted, of the height the staff MNis held. Run the cursor C D to the third great division of the staff, as at G ; then ap-proaching or retiring from the piquet L, holding the staff’ horizontal, and at the height ofthe piquet L, bone from the end of the staff M, by the two extremities C and D of thecursor, until the foot A and the summit B are seen, which will be at the station K,where a piquet must be planted for the second station. Then, measure in a right linethe distance between the two piquets K, L, which will lie the height required; that is tosay, if the distance KL is 80 feet, the height AB will be 80 feet: small heights canonly be measured by this means, as the instrument must be placed opposite the middleof the height required.
To measure Lengths when they are accessible about midway. — Stand opposite the middle*of the required length, and dispose the staff and cursor parallel to the horizon; bonethe two objects by this end of the staff'and the two extremities of the cursor, whichshould be placed parallel to the distance be-tween the two objects; this may be done bymoving the cursor nearer to or further fromthe eye: count how many small divisionsthere are on the rod between the end ofthe staff boned and the place where thecursor stopped ; measure how far it is fromthe station whence the two objects wereboned to the middle of the line drawnbetween them: by the rule of three placeas a first term the number of small partsat which the cursor stopped on the rod ;in the second term the number of feetfrom the station to the middle of the dis-tance between the objects ; and in the third,
30 for the length of the cursor : the quotientwill be the answer. The distance fromA to B is required, which to obtain wemust place ourselves at the station K, oppo-site the middle of the distance between Aand B, and dispose the staff in such a way that the rod and cursor may be parallel tothe horizon, in order that, when boning from the end of the cursor C and D, the twoobjects A and B may be discovered, either by advancing or retiring the cursor back, as to the60 division. Then measure how far it is from the station K to the point L, which isthe middle between the two objects A and B, or 450 feet. Then by the rule of three,placing as a first term 60, the number at which the cursor stands, and in the second term450, the number of feet from the station K to the middle L, between the two objects