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adjustment angular axis B A C back-sight barometer bisect the target bottom bring the bubble bubble-tube centre circle clamp column contour-lines correction curvature datum desired height determined difference of heights direction Dumpy Level end for end equal error eye-piece feet fore-sight formula gamboge given ground half-way back hold the rod horizontal inch justment length level line line of collimation line of sight lower one end main socket mark means measured mercury method miles millimetres mirror move note the reading object object-glass observed angle parallel plate perpendicular placed plane of reference plumb-line precisely radius raise or lower right angles rod held screws set the instrument sextant side slope spherical angles spherical excess stakes starting-point station surface Surveying taken temperature thalwegs theodolite tical tion triangle tripod tube turn the telescope turning-points upper usually vernier vertical angles vertical plane wyes zenith distance
Page 144 - All buoys along the coast, or in bays, harbors, sounds, or channels, shall be colored and numbered, so that passing up the coast or sound, or entering the bay, harbor, or channel, red buoys with even numbers shall be passed on the starboard hand, black buoys with uneven numbers on the port hand, and buoys with red and black stripes on either hand. Buoys in channel-ways shall be colored with alternate white and black perpendicular stripes.
Page 1 - ... a curved line, because it is parallel to the curvature of the sea. But for all ordinary purposes a level line and a horizontal line are synonymous even for leveling operations conducted over such great distances as to be affected by the curvature of the earth. A level surface may also be defined as one which is everywhere perpendicular to the direction of gravity as indicated by a plumb-line; and the spirit-level, like the plummet, is a device for utilizing the law of gravity to estab lish a...
Page 12 - ... that a longitudinal section is a segment of a circle. If the tube is not ground to an an even curvature the bubble will not travel the same distance for every minute of arc to the extreme ends of the tube, and an otherwise perfect instrument will not work well. Fig. C. A line tangent to the circular arc at its highest point, as indicated by the middle of the bubble, or a line parallel to this tangent, is called the axis of the bubble tube. This axis will bo horizontal when the bubble is in the...
Page 166 - ... of the true spherical excess, the sines of these angles will be proportional to the lengths of the opposite sides, and the triangle may therefore be calculated as if it were plane.
Page 14 - Otherwise, if the bubble.tube is capable of movement, raise half.way back to the middle by this means, and the other half by raising or lowering one end of the block, because the reversion has doubled the error. Repeat this, if necessary. Circular LeveL The upper surface of this is spherical. It will therefore indicate a level in every direction, instead of only one, as does the preceding. It is adjusted like the last one, but in two directions, at right angles to each other.
Page 43 - Steep Slopes. In descending or ascending a hill, the instrument and the rod should be so placed that the sight should strike as near as possible to the bottom of the rod on the up-hill side, and the top of the rod on the down-hill side. Try this by levelling over two screws, setting the instrument so that one pair of opposite plate-screws shall point in the direction of the line, but do not be too particular ; it is a waste of time.
Page 132 - For higher or lower temperatures add or subtract 1| foot for each degree of Fahrenheit. If a wind blows with or against the movement of the sound, its velocity must be added or subtracted. If it blows obliquely, the correction will evidently equal its velocity multiplied by the cosine of the angle which the direction of the wind makes with the direction of the sound.* If the gun be fired at each end...
Page 119 - The line of collimation of the telescope is an imaginary line, passing through the optical centre of the object-lens, and a point midway between the two parallel wires. These wires are made parallel to the plane of the sextant by revolving the tube in which they are placed. To see whether the line of collimation of the telescope is in adjustment, bring the images of two objects, such as the sun and moon, into contact at the wire nearest the instrument, and then, by moving the instrument, bring them...
Page 73 - ... water are determined by the difference of the depths below the top of the water. The desired height of the atmosphere above any point, such as the top of a mountain, or the bottom of a valley, is determined by weighing it. This is done by trying how high a column of mercury or other liquid the column of air above it will balance ; or what pressure it will exert against an elastic box containing a vacuum, etc., etc.