Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . pansion; the protection of thelevel-vial and the middle part ofthe telescope from sudden andFig. looa.—Section of Binocular unequal changes of temperature byPrecise Level. incasing them in an outer tube; and an arrangement by which, without any change of the observersposition, the level-bubble can be clearly seen by his left eye atnearly the same instant in which the distant rod is observedthrough the telescope by his right eye. It has been found that one
Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . pansion; the protection of thelevel-vial and the middle part ofthe telescope from sudden andFig. looa.—Section of Binocular unequal changes of temperature byPrecise Level. incasing them in an outer tube; and an arrangement by which, without any change of the observersposition, the level-bubble can be clearly seen by his left eye atnearly the same instant in which the distant rod is observedthrough the telescope by his right eye. It has been found that one of the principal sources of error insome of the precise leveling of the past was due to unequal changesof temperature in the telescope and parts connecting it with thelevel-vial. The momentary changes in the relative positionsof the level-vial and telescope produced by these temperaturechanges, though microscopic in magnitude, were yet sufficientto introduce appreciable errors into the leveling on long particular nickel-iron alloy used in the construction of thenew level expands less than one-fourth as much as brass for a. PRECISE SPIRIT-LEVEL. 327 given increase of temperature. This, together with the factthat the level is mounted as close as possible to the line of sight,gives a very high degree of stability to the relation between thetwo. 142. Precise Spirit-level.—The adjustments of precise levelsdo not differ essentially from those of ordinary Y levels. In
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