Where a long length is laid down for a base line or other purpose, it is better to take the thermometer reading at each measurement and defer correction to the completion of the work; the temperature errors may then be added together as a total, and the space allowance may become a measurable quantity. For example, say ten 10-feet lengths give by these united centigrade degrees, plus and minus, shown at separate readings + 167°, and that the standard of the rods is true at 15·5°. Then 167 - (10 × 15·5) = + 12° per foot total allowance, that is, 12° × 10 feet × ·000019 = ·0228 feet or ·2736 inches to be added. In measuring iron of course no correction has to be made.
756.—Beam Compass Measurements are occasionally preferred for iron work. In this case the beam is moved from centre punch mark to mark along a surface by the beam producing a scratch for the forward position in which to place the punch mark. Rods of pine are commonly employed. Figs. 357, 358 will sufficiently illustrate the instruments.
Fig. 357.—Beam compasses.
Fig. 358.—Standard scale.
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Fig. 359.—Coincidence rods.
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757.—The Method of Coincidence in measurements by rods has often been applied to measurement of base lines. The plan consists in allowing one rod, or a lighter continuous part of it, to pass the other rod, so that a line cut to standard on one rod may be read into one on the other. The best plan to do this is to have a scale fixed along the face of one rod near its end, as shown Fig. 359, and to have an extension from the other end of the second rod to pass alongside this scale, so that two lines may be brought into coincidence. The rod B has a fixed scale b placed on top of it at one end. The rod A has a scale protruding from it. This scale may be jointed with a good ground joint at J for portability. The rods are laid lightly together, and any final adjustment is given by light taps with a small hammer or mallet upon one or the other side of the stud P until exact coincidence of the lines shown at b is brought about. This tapping operation appears a rather rough process, but practically it is very exact.