A Treatise on Military Surveying: Including Sketching in the Field, Plan-drawing, Levelling, Military Reconnoissance, Etc. : Also a Particular Description of the Surveying Instruments Commonly Employed by Military Men, with Instructions for Using and Adjusting Them

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W.H. Allen, 1853 - 360 pages
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Page 229 - ... but it may be made in a variety of ways, so as to revolve on any light portable stand. The tube, when required for use, is filled with water (coloured with lake or indigo), till it nearly reaches to the necks of the bottles, which are then corked for the convenience of carriage. On setting the stand tolerably level by the eye, these corks are both withdrawn, which must be done carefully and FRENCH REFLECTINQ-LEVEL.
Page 206 - THE first adjustment is that of the line of collimation; that is, to make the intersection of the cross wires coincide with the axis of the cylindrical rings on which the telescope turns : it is known to be correct, when...
Page 208 - ... vane being raised or lowered by the assistant, until the cross wire corresponds .with the horizontal wire of the telescope, the height of the wire in the vane, noted on the staff, is the height of the apparent level above the ground at that place. When both the staves are used, they should be set up at equal .distances on each side of the spirit-level : the difference of the heights of their vanes •will be the absolute difference of level between the two stations. But when one staff only is...
Page 88 - ... and moving the index bring the image of the object to appear with the levels, which must have their air-bubbles in the centre of their tubes. The reading of the instrument will then show the supplement of the zenith distance, and its complement to 90 will be the angle required ; elevated, if more than 90, and depressed, if less than 90.
Page 206 - First, to place the intersection of the wires in the telescope, so that it shall coincide with the axis of the cylindrical rings on which the telescope turns; secondly, to render the level parallel to this axis; and lastly, to set the telescope perpendicular to the vertical axis, that the level may preserve its position while the instrument is turned quite round upon the staves. To Adjust the Line of Collimation. The eye-piece being drawn out to see the wires distinctly, direct the telescope to any...
Page 207 - ... half the error must be corrected by turning the screw, B, and the other half by the two parallel plate-screws, over which the telescope is placed. Next turn the telescope a quarter round, that it may lie over the other two screws, and make it level by moving them ; and the adjustment will be complete. Before making observations with this instrument, the adjustments should be carefully examined and rectified, after which the screw, B, should never be touched ; the parallel plate-screws alone must...
Page 297 - What is the error of the best tables now in use ? hours, or one minute of space in two minutes of time. Therefore, if we make an error of one minute in observing the distance, we make an error of two minutes in time, or 30 miles of longitude at the equator. A single observation with the best...
Page 119 - ... in chains and decimals. Therefore, after the content is found, it will be in square links ; then cut off five of the figures on the right hand for decimals, and the rest will be acres. These decimals are then multiplied by 4 for roods, and the decimals of these again by 40 for perches.
Page 106 - ... the number of links to be deducted from each chain's length, in measuring up or down an inclined plane, to reduce it to the horizontal measure.
Page 266 - ... 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 the...

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