Fig. 351.—Jointed offset rod, top and centre.
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749.—Offset Rods are generally made 10 links long, either in one piece or jointed in the centre with a bayonet joint. They are about 1-1/8 inches in diameter, diminishing towards the top to 7/8 inch, and made either of yellow pine or ash. A hook is commonly put at the top, Fig. 351, which takes the handle of a chain to draw it through a hedge or other obstruction. The author's plan of making this is shown at H. The lower end of the offset is shod with a steel or wrought iron socket point, so that it may be set up in the ground and used if required as a picket. Bands are painted alternately black and white at every link. Square or flat rods are occasionally used for the same purpose, but they are not generally so convenient.
The offset is Used in the manner of an ordinary rule to take rectangular short measurements from the chain as it lies upon the ground, commonly in order to obtain the contour of irregular outlines.
750.—Measurement by Rods has become less general than formerly, from the greater accuracy of Konstat or Invar steel tapes, by which practically correct base lines may be laid down. For geodetic works requiring the greatest accuracy the bases have been laid with rods of various forms. These rods will be briefly described. It is only in the construction of iron bridges, roofs, etc., that rods are at present generally employed in the work of the civil engineer.
751.—Pine Standard Rods, made of straight-grained pinewood seasoned five or six years and then well soaked in linseed oil, make good standard rods. The ordinary length in use is 10 feet by 1¾ inches square. If the rod be used for butt measurement the ends are tipped with gun-metal in which a turned steel stud is hard-soldered. The stud is afterwards ground to true face in a lathe, and left of standard length at 60° Fahrenheit (15·5 centigrade), Fig. 352. A disc of brass 1 inch diameter is inlaid at every foot for 5 feet from one end of the rod, with a line at the true foot. These rods, after the work upon them is finished, are lightly French polished to keep them clean and to prevent the effects of moisture. The effect of temperature upon deal was found by Roy to be about the same as upon glass—·0000085, average of total length per degree centigrade, which is about three-fourths that of iron.
Fig. 352.—One end of a pinewood butt rod.