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A lens which introduces astigmatism into an optical system. Such a lens is so arranged that it can be placed in or removed from the optical path at will. In a sextant, an astigmatizer may be used to elongate the image of a celestial body into a horizontal line.
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Related Terms |
PLUNGE
To set the horizontal cross hair of a theodolite in the direction of a grade when establishing a grade between two points of known level.
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PENDULUM SCALE
Weight-measurement device in which the load is balanced by the movement of one or more pendulums from vertical (zero weight) to horizontal (maximum weight).
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PENDULUM LEVEL
A leveling instrument in which the line of sight is automatically kept horizontal by a built-in pendulum device (such as a horizontal arm and a plumb line at right angles to the arm).
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PENDULUM ANEMOMETER
A pressure-plate anemometer consisting of a plate which is free to swing about a horizontal axis in its own plane above its center of gravity; the angular deflection of the plate is a function of the wind speed; this instrument is not used for station measurements because of the false reading which results when the frequency of the wind gusts and the natural frequency of the swinging plate coincide.
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OPTICAL BENCH
A rigid horizontal bar or track for holding optical devices in experiments; it allows device positions to be changed and adjusted easily.
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NEEDLE DAM
A barrier made of horizontal bars across a pass through a dam or of planks that can be removed in case of flooding.
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MOVABLE BRIDGE
A bridge in which either the horizontal or vertical alignment can be readily changed to permit the passage of traffic beneath it. Often called drawbridge (an anachronism).
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MAXIMUM GRADABILITY
Steepest slope a vehicle can negotiate in low gear; usually expressed in precentage of slope, namely, the ratio between the vertical rise and the horizontal distance traveled; sometimes expressed by the angle between the slope and the horizontal.
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LONGITUDINAL STABILITY
The ability of a ship or aircraft to recover a horizontal position after a vertical motion of its ends about a horizontal axis perpendicular to the centerline.
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LONGITUDINAL DRUM BOILER
A boiler in which the axis of the horizontal drum is parallel to the tubes, both lying in the same plane.
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