. Machinery and processes of the industrial arts, and apparatus of the exact sciences. ertical plane in which theobject is situated of which the altitude is to be measured; and by turn-ing the telescope in its supports the image of the object is seen byreflection in the prism. At the extremity next the eye. the telescopecarries a vertical circle of ten centimeters (four inches^ radius, divided THEODOLITES LEVELING INSTRUMENTS. 597 centesimally, and this is read by a vernier to one one-hundredth partof a degree. The horizontal points of the limb are not determinable bydirect observation; but th
. Machinery and processes of the industrial arts, and apparatus of the exact sciences. ertical plane in which theobject is situated of which the altitude is to be measured; and by turn-ing the telescope in its supports the image of the object is seen byreflection in the prism. At the extremity next the eye. the telescopecarries a vertical circle of ten centimeters (four inches^ radius, divided THEODOLITES LEVELING INSTRUMENTS. 597 centesimally, and this is read by a vernier to one one-hundredth partof a degree. The horizontal points of the limb are not determinable bydirect observation; but the zen-ith and nadir points are foundwith facility. For the nadirpoint resort is had, as usual, toreflection from the surface ofmercury. For the zenith, atrough filled with water, andhaving as a bottom a plate ofglass with parallel surfaces, isplaced over the object end ofthe telescope. The upper sur-face of the water serves as areflector. The zenith and nadirpoints being found, their circlereadings, if correct, will differby a semi-circumference. Andit is easy at any time to verify. to Dabbadies Theodolite. their accuracy by observing the zenith distance of any object, and alsothat of its image seen by reflection in mercury, and adding the sum should be a semi-circumference. The horizontal circle is ofthe same diameter as the vertical. The movements, both horizontaland vertical, are effected without tangent-screws, the rack and pinion onlybeing used. Two cross-levels serve for the adjustment of the plane ofhorizontal movement. Mr. Eichens has also added a small compass tothe instrument exposed by him. Nothing could be in appearance moreconvenient than this instrument for the use of the scientific travelerwho desires to make observations upon the heights of mountains, thebreadths of streams, or other matters of geographical or geodeticalinterest. LEVELING INSTRUMENTS. Leveling instruments of excellent construction were exhibited by Tavernier of Paris, and by Mr
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Keywords: ., bookcentury1800, booksubjectmachinery, booksubjectscientificappa
