. A treatise on surveying and navigation: uniting the theoretical, the practical, and the educational features of these subjects. e three adjustments are now made approximately, no one ofthem can be made perfectly while the instrument is greatly out ofadjustment in relation to the others ; therefore commence anew.—Bring the bar EE over two of the leveling screws, and level theinstrument; then turn it over the two other screws and level it in thatdirection also. Now, if we can turn the instrument quite round LEVELING 161 without removing the bubble from the center it is in pretty good ad-justme
. A treatise on surveying and navigation: uniting the theoretical, the practical, and the educational features of these subjects. e three adjustments are now made approximately, no one ofthem can be made perfectly while the instrument is greatly out ofadjustment in relation to the others ; therefore commence anew.—Bring the bar EE over two of the leveling screws, and level theinstrument; then turn it over the two other screws and level it in thatdirection also. Now, if we can turn the instrument quite round LEVELING 161 without removing the bubble from the center it is in pretty good ad-justment, but if otherwise, as is to be expected, make all theseadjustments over again; they can now be made with much lessdifficulty. It is important that a level should be in as perfect adjustment aspossible, but perfection in all respects is almost, yea, quite impossi-ble. Yet, with a level considerably out of adjustment, we can ob-tain the relative elevation of any two points, provided we can set thelevel midway between them. To illustrate this, suppose the instrument placed at D, midwaybetween two perpendicular rods Aa Let ah represent the true horizontal line, but suppose that the in-strument is so imperfect, or out of adjustment, that when the level-ing tube CD is horizontal, the telescope would point out the risingline DA, and the rise would be Aa. On turning the instrumentround and sighting to B, the rise must be the same as in the oppo-site direction: for the distance is the same, therefore A and B are astruly on a level with each other as a and b. By this problem, practical men complete the second adjustment ofthe instrument. They make the three adjustments as just explained,as accurately as possible. They then measure, very carefullythe distance between two stations, as E and F, and set the in-strument exactly midway between them as represented in the lastfigure. They then level the instrument ( that is the tube CD ), andfind the difference of the levels between E
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Keywords: ., boo, bookcentury1800, booksubjectnavigation, booksubjectsurveying
