Practical Geodesy: Comprising Chain Surveying, and the Use of Surveying Instruments; Leveling, and Tracing of Contours; Together with Trigonometrical, Colonial, Mining, and Maritime Surveying. Adapted to the Use of Surveyors, and of Students in Civil and Military Engineering

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J.W. Parker, 1846 - 330 Seiten

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Seite 10 - Upon the same base, and on the same side of it, there cannot be two triangles that have their sides which are terminated in one extremity of the base equal to one another, and likewise those which are terminated in the other extremity.
Seite 121 - ... near each of their extremities, a fine steel pricker, the two points of which, and the centre of the protractor, must (for the instrument to be correct) be in the same straight line. The points are prevented from scratching the paper as the arms are moved round, by steel springs, which lift the branches a small quantity, so that, after setting the centre of the protractor over the angular point, and the vernier in its required position, a slight downward pressure must be given to the branches,...
Seite 270 - ... to make the objects appear on the other wire ; if the contact still remains perfect, the axis of the telescope is in proper adjustment ; if not, it must be altered by moving the two screws which fasten, to the up-and-down piece, the collar into which the telescope screws. This adjustment is not very liable to be deranged.
Seite 316 - EBD) or cot.(180°-lBED-EBD), whence the angle EDB is given. The angles which all the three objects, A, B, C, subtend at the point D are therefore all given, and hence the position of D is determined by the preceding proposition. But BD being found, the several distances BE, ED, and BF, FD, are thence obtained, and consequently the position of each of the stations E and F is determined.
Seite 278 - Suppose, now, two observers, at distant stations, A and B, each independently of the other, to set and regulate his clock to the true sidereal time of his station. It is evident that if one of these clocks could be taken up without deranging its going, and set down by the side of the other, they would be found, on comparison, to differ by the exact difference of their local epochs; that is, by the time occupied by the equinox, or by any star, in passing from the meridian of A to that of B : in other...
Seite 268 - ... Horizon-glass, and set it perpendicular to the Plane of the Sextant. The position of this glass is known to be right, when by a sweep with the index, the reflected image of any object passes exactly over or covers its image, as seen directly; and any error is easily rectified by turning the small screw, i, at the lower end of the frame of the glass. To examine the Parallelism of the Planes of the two Glasses, when the Index is set to Zero. This is easily ascertained ; for, after setting the zero...
Seite 58 - ... alone. Now the errors of observation, when numerous, tend to balance and destroy one another: so that, if sufficiently multiplied, their influence will disappear from the result. There remains, then, only the constant error of graduation, which comes to be divided in the final result by the number of observations, and is therefore diminished in its influence to one-tenth of its possible amount, or to less if need be.
Seite 268 - ... middle of the limb, then, holding the instrument horizontally with the divided limb from the observer, and the index-glass to the eye, look obliquely down the glass, so as to see the circular arc, by direct view and by reflection, in the glass at the same time ; and if they appear as one continued arc of a circle, the index-glass is in adjustment. If it requires correcting, the arc will appear broken where the reflected and direct parts of the limb meet. This, in a well-made instrument, is seldom...
Seite 117 - When the plumb-line, the star Alioth, and the north star, fall on the .vertical spider's line, the horizontal limb is firmly clamped, and the telescope brought down to the horizon ; a light, seen #' through a small aperture in a board, and held at some distance by an assistant, is then moved according to signals, until it is covered by the intersection of the spider's lines. A picket driven into the ground, under the light, serves to mark the meridian line for reference by day, when the angle formed...
Seite 190 - ... the height of every embankment and the depth of every cutting, and a datum horizontal line, which shall be the same throughout the whole length of the work...

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