. A treatise on surveying and navigation: uniting the theoretical, the practical, and the educational features of these subjects. 95 21 .552 46 71 96 22 .605 47 72 97 23 .661 48 73 98 24 .720 49 74 99 25 .781 50 75 100 This table is of little or no practical use, for levelers rarely takesight to a greater distance than 10 chains, and at that distance thecorrection is only one-eighth of an inch, and if they put the levelmidway between two stations, they annihilate the correctionsaltogethe
. A treatise on surveying and navigation: uniting the theoretical, the practical, and the educational features of these subjects. 95 21 .552 46 71 96 22 .605 47 72 97 23 .661 48 73 98 24 .720 49 74 99 25 .781 50 75 100 This table is of little or no practical use, for levelers rarely takesight to a greater distance than 10 chains, and at that distance thecorrection is only one-eighth of an inch, and if they put the levelmidway between two stations, they annihilate the correctionsaltogether. Suppose a level to be placed at T, midway between A and B; theinstrument will show them to be on the same level, as so they reallyare, for they are at equal distances from the center of the earth ; butif the observations were taken in reference to A and a, the apparentlevel would not show equal distances from the center of the earth,and a correction must be applied, if the difference of distances ismore than four or five chains. 158 SURVEYING. To comprehend the whole subject, we must now describe themodern SPIRIT The figure before us represents this useful instrument, apart fromits tripod. Its principal parts are the telescope AB, to which is attached theleveling tube CD; the telescope rests in a bed, which is supportedby posts yy, called the ys; EE is a firm bar, supporting the S is a socket, which recetves the central pivot of the tripod(which is not here represented). When the instrument is put upon its tripod, the tube S can beclasped on the outside, and held firmly by a clamp screw, it canthen be moved horizontally, as minutely and readily as desired, bymeans of a tangent screw. The tripod contains a pair of brass plates, to the lower one thelegs of the tripod are firmly attached, the other plate moves in alldirections on its center, and is worked by four screws; these arecalled the leveling screws; these plates are purposely made small asa greater surety
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Keywords: ., boo, bookcentury1800, booksubjectnavigation, booksubjectsurveying
