800
THEORY AND PRACTICE OF ENGINEERING.
Book II.
pressed firmly into the ground at each station,be much change apparent in the level taken.
When it is required to know whether thepoints E, E, are elevated or lower than thepoints C and G, place piquets at D, E, andhaving properly rectified the level, place itagainst the piquet at B C, and bone the line Ethrough the telescope, and the card must beraised or lowered until the visual ray meetsthe centre of it at E; then measure theheight that the centre of the eye-glass at C isabove the base B, and set off from the base ofthe piquet D this same height B C, and theheight then up to the point at E on the card,will be the difference of the levels.
If the observed height fall lower than thesight point, as at K, it proves that the groundis lower than at I, as much as the difference isbetween K and II.
When it is required to find the differencebetween the levels of the spring at A andanother at B, piquets of the required lengthmust be fixed at the stations 1), C, and B, andthe level may he placed at C; then by takingthe necessary observations, measuring off theheights on the piquets, and deducting the heightof the stand, it may be easily computed ; or, sup •pose the height from B to F to be 16 feet, andthat of A D 10 feet, the difference will be thevariation in the level.
If it be required to ascertain whether awell at the station at F is above or belowthe level at B; after rectifying the level,place it at D, where both piquets can beseen : when the observations have been made onthe piquets, their difference can easily be cal-culated, which will be the variation in the level.
If it be necessary to plant piquets in a linefrom the trunk of the tree A to the descentat B, where hills as F, G, and II, intervene; apiquet must be placed at A D, and another atB E, and then the level must be stationedbetween the two objects A B, at any placewhere the piquet A D can be seen.
From the station F, bone the piquet A D,and then looking through the other end ofthe level place in a line the piquets Q, R, S ; andif this be found not to cut the piquet B, thenfrom the last of these piquets bone R and Q,and take up another station, as at G, boningthrough the piquets P, O, N ; we must continueour observations until by boning from thepiquets A D and B E, we find B E in the visualray with the other piquets; then a line drawnthrough the foot of the piquets planted on thehills and valleys between the two stations Aand B, as A, N, O, P, and B, will mark theshortest road required.
When the district through which the levelsare to be taken is much intercepted by treesor buildings, it is often found very useful toleave at convenient intervals marks which maybe cut in posts, stumps of trees, or painted ona fence or building : such a bench-mark willbe valuable for future reference, and during thesurvey to check the levels made in differentdirections; and it should always be establish-ed at the end of every day’s labour.
and then on turning the face, there will not
A
I'ig. 927
Fig 92*.
Fig. 929.
Fig 931.