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tween two objects Y and D, it is evident the effect of curvature and refraction in both will be the same: consequently the difference of level of the places Y and D, is obtained by taking the difference of the apparent levels. In the common practice of levelling for roads or canals, neither curvature or refraction is allowed for; as the dis'tance between the instrument and either station is in general so small, that no appreciable error can arise, the distance being but a few yards. But it would be advisable to plant the instrument as near the middle between the stations as you could guess at..
We cannot proceed farther with the principles of leveling, until we have described the instruments which are to be used, and explain the object they are to answer with their adjustments.
First of the theodolite. This instrument we have already fully described.
The Y Level.-This instrument has its name from the resemblance its supports bears to the letter Y. Its adjustments are convenient to be performed, but on the other hand, it seldom retains its adjustments perfect for any length of time; besides, there are conditions in its construction which presents difficulties in the manufacture. The first adjustment in this level is the line of collimation. This adjustment is performed exactly like that of the telescope in the theodolite. Open the rings that confine the telescope, draw out the eye-piece to see the wires distinctly, direct the telescope to some object, and adjust it so that the cross-wires and object may be seen with perfect clearness: then bring the intersection of the cross-wires to coincide with some well-defined part of the object, then clamp
the instrument firm, turn the telescope round in its (Y's) and if the bisection continue during an entire revolution, the line of collimation is properly adjusted. But should not the cross-wires continue to bisect the object, the wires must be moved one-half the quantity of error, by means of the little screws near the eye end of the telescope, which carry the cross-wires, one of which must be loosened before the opposite one is tightened; which, if perfectly done, will perfect the adjustment; if not you must repeat the operation until the adjustment is perfect.
2. To set the level parallel to the line of collimation.
Move the telescope till it lies in the direction of two of the parallel plate-screws, and bring the bubble to the centre of the level by means of the parallel plate-screws; then reverse the telescope in its Y's, and if the bubble return to the middle of the tube, all is right; but if not, correct half by means of the screws which connect the tube to the telescope, (called the capstan-head screws,) and correct the other half by means of the parallel plate-screws. Again, reverse the telescope, and observe whether the bubbles returns to the middle of the tube, if not, the correction must be repeated as before directed until the adjustment is perfect.
3. The next adjustment is to make the instrument revolve on its stand-the bubble remaining in the middle all the while.
Place the telescope over two of the parrallel plate-screws, then bring the air-bubble to the middle of its tube; then turn round the instrument on its stand, until the contrary end of the telescope is over the same two screws, and if the bubble returns to the middle of the tube again, all is right in that position; but if not, raise or lower the milledhead screw, which carries one of the Y's, one half the error, and correct the other half by means of the parallel plate-screws. Next turn the instrument a quarter round,
that it may lie over the other two screws, and make a level by the help of them, proceed here as before: continue the correction till the air-bubble will remain in the middle of the tube during an entire revolution. Before making observations with this instrument, the engineer should carefully examine the adjustments and rectify if any
In describing the method of adjusting the theodolite, it was recommended to attend, first, to the eye-piece of the telescope. This is the first adjustment to be attended to in the levelling instruments.
Having adjusted the eye-glass, the next is to adjust the object-glass then set the instrument level by means of the parallel plate-screws.
Troughton's Improved Level.
This modification of the instrument has every decided advantage over the Y level: in its construction it is more compact, and the adjustment when once made are less liable to be deranged. In this instrument, the spirit-level is fixed to the telescope, and therefore it has no separate means of adjustment. The telescope is also firmly fixed. in its frame, and cannot be reversed. Hence, the line of collimation must depend on the spirit-level.
The adjustment: suppose the instrument to be set up at B, (Fig. 16,) then level the telescope by means of the parallel plate-screws; set up a staff at the distance of a few chains as at A, upon a good level; let B K be the height of the instrument 4 feet, A S the cut on the pole, or height of the vane, suppose 8 feet; their difference=4 feet A Z., then let the instrument and staff change places, and observe the height as before, and if it gives the same difference of level as the last, the instrument is correct: but if the instrument is incorrect, the direction of the line of sight will be either above or below C D as CO or C
N: if a above, the difference of level will be greater than 4, and consequently the vane must be lowered half the quantity, and the collimation-screws moved to correct the other half; if the sight falls below as Co, the difference will be less than 4 feet, and the vane must be raised half the quantity, and the other half corrected by the collimation-screws which gives motion to the wires.
There is a practical difficulty in finding the true difference of level between these two last points A and B, by merely causing the instrument and staff to change places. This difficulty may be done away with in the following manner: choose some level ground, as (Fig. 17,) upon which set up the instrument at C, and level it by means of the parallel plate-screws; then place the staff at A a few chains from the instrument, and read off the height cut by the intersection of the cross-wires: place the staff the same distance on the other side as at B, and read off again the difference of these two readings will be the true difference of the level between the two places A and B, notwithstanding the error at the same time existing in the instrument. This is evident, as the angle t my is = angle s m x (allowing t m s horizontal,) and t m=ms.'.sx =ty and At=os; that is, Ay is=ox: Hence, Ay-B x =Bo, the true difference of level between A and B, (having A o horizontal.)
Now remove your instrument to the point P a few feet from B, and note the reading on the staff placed alternately on the stakes, and if both readings gives the same difference of level as before, theo ptical axis of the telescope is right; but if not, alter the position of the wires by means of its screws, until the difference of the cuts gives the exact difference of level, the same as from the point C. Now turn the instrument round on the staff-head, and observe if the bubble retains its position in the middle of the tube, if so, all is right; if not, lower the end to which the
bubble approaches, as in the adjustment of the Y level; thus correcting half the error by means of the capstanhead screws attached to the end of the horizontal bar, and the other half by the parallel plate-screws. After every correction, the instrument must be tried, and if any error be found to exist, the process of correction must be repeated.
Another good method of adjustment is by means of a sheet of water: (Fig. 18,) thus, drive two stakes close to the water's edge, so that the upper ends may be even with the surface of the water, as at n and m; let the level be set up at A a few chains from the stakes; by the help of the parallel plate-screws bring the bubble to the centre of the tube. Now read off the staff placed on each stake, and if the cuts be the same, all is right; if not, move the horizontal wire up or down, until the cuts are exactly the same height on both staffs.
(You are to observe, this method is only good to adjust the cross-wires, when spirit-bubble is truly adjusted.)
The description, use, and adjustments of Gravett's Level, commonly called the Dumpy Level.
Mr. Gravett's modification of the spirit-level is praiseworthy, whereby he obtains advantages, both optical and mechanical. The only disadvantage attending it, is the difficulty in adjusting its several parts. But indeed, this difficulty of adjustment is more than counter-balanced by the many advantages peculiar to it.
The object-glass is of a large aperture and a short focal length to the telescope, for the purpose of obtaining the light and power of a large instrument without the inconvenience of its length. By it we can read within the range of practice; thus avoiding the tedious and disagreeable practice of communicating intelligence to the assistant by signs, and also guarding against the errors consequent on his neglect. This instrument inverts objects,