776.—Slip Jointed Rod.—This form is less general, but it is a very convenient form of rod. The jointing is effected by two loops which are fixed to the centre end of one part of the rod in such a manner that the other part may slide through the loops. When the rod is extended to 5 feet there is a stop which prevents further extension, and a spring to keep it at this exact position, Fig. 381. The outside of the rod is divided into feet and inches. The inside is divided so that any addition to the half rod, produced by extending it; may give the measurement from end to end of the rod at this position, thus:—The half rod being 2 feet 7 inches closed, if the loose side be drawn out 19 inches the rod from end to end will be 4 feet 2 inches, which will be indicated by the division and figuring inside the rod. This is very convenient for measuring openings such as doorways or passages.
777.—Brace-piece.—A 10-feet rod is sometimes made with a brace-piece, which folds up inside the rod. This brace-piece is jointed to fix both half rods to 90° when it is desirable to use the rod as a square.
Fig. 382.—Civil engineer's rule.
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778.—Civil Engineer's Rule is made fourfold in both boxwood and ivory, Fig. 382. The most convenient size is 1 inch wide. Some of the profession prefer them narrow for lightness—¾ inch; and some wide for strength—1¼ inches. This rule is generally well made, with German-silver joints and outside joint-plates. The divisions placed on the rule outside are inches in eighths and tenths; the inside, the ordinary architects' scales, 1/8, ¼, ½, 1, and four chain scales, 20, 30, 40, and 50. A 10 is got by halving the 20; 60, by doubling the 30. A protractor reading to 5° is divided on the head. With silver joints and in fine ivory this rule is often made a presentation instrument.
CHAPTER XVII.
STATIONS OF OBSERVATION—PICKETS—FALSE PICKET—PERMANENT STATIONS—REFERRING OBJECT—HELIOTROPE—HELIOSTAT—HELIOGRAPH—SIGNALLING—MORSE ALPHABET—NIGHT LIGHTS—OIL LANTERNS—MAGNESIUM LIGHT.