Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . a transverse motion, and an elevating-screw for final adjustment of the level. These tripods reston trestles which are at various heights according to the natureof the ground. The interval between two adjacent compen-sating points lying in a line is brought to exactly 6 inches bymeans of a compensation microscope. 210. Contact-slide Base Apparatus.—The contact-slide base-measuring apparatus, made by Saegmuller & Co.,consists of two measuring-bars, each 4 m
Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . a transverse motion, and an elevating-screw for final adjustment of the level. These tripods reston trestles which are at various heights according to the natureof the ground. The interval between two adjacent compen-sating points lying in a line is brought to exactly 6 inches bymeans of a compensation microscope. 210. Contact-slide Base Apparatus.—The contact-slide base-measuring apparatus, made by Saegmuller & Co.,consists of two measuring-bars, each 4 meters in length andsupported on trestles. (Fig. 155.) ^\i& measurement is madeby bringing these bars successively in contact, which is effectedby means of a screw motion and defined by the coincidenceof lines on the rod and contact-slide. Each bar consists oftwo pieces of wood about 8 x H cm. square and a little lessthan 4 meters long, firmly screwed together. Between thepieces of wood is a brass frame carrying three rollers, on thecentral one of which rests a steel rod about 8 mm. in diam- CONTACT-SLIDE BASE APPARATUS. 509. 5IO FIELD-WORK OF BASE MEASUREMENT. eter. On each side there is a zinc tube 9 mm. rod and tubes are supported througliout their length onsimilar systems of rollers. The zinc tubes form with the steelrod a metallic differential thermometer, and are so arrangedthat one tube is secured to one end of the rod, being free toexpand in the other direction, the other tube being in a likemanner fastened to the other end of the rod. The zmc tubes,therefore, with any change of temperature, expand or contractin opposing directions, and the amount by which the expan-sion of the zinc exceeds that of the steel is measured by afine scale attached to the rod, while the zinc tube carries acorresponding vernier. The cut shows this arrangement,which is identical on both ends of the bars; a perforation inthe wood of the bar allows this scale to be read. In additio
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Keywords: ., book, bookcentury1900, bookdecade1900, bookpublishernewyorkjwiley
