Page:The New International Encyclopædia 1st ed. v. 07.djvu/98

* ENGINEERING. parallel to the one which it occupied in starting irvey. Holes are provided at one end of the board for holding pencils. A rough cli- r has been added at the other end. The of maps made in the field is usually selected between one inch to the mile and six inches to the mile. Two inches to the mile has been found convenient for reconnoissances made on horse- back, and four inches to the mile for foot recon- noissances. The scale used should be clearly shown on the map. A north-and-south line, the name of the reconnoitrer, and a clear title should also be plainly indicated on it. Where the primary object of the survey is to a road-map, the road is gone over by the i oitrer and mapped directly on the paper. He indicates also, as a rule, the position of the prominent objects within range of the road, noting the names of places. Collateral informa- tion as to distances, water-supply, camping- grounds, food, fuel, etc.. may also be given. I oder other conditions, positions favorable for <■ and defense are noted. The character of the roads so far as these may affect the move- ment of troops is recorded. Information rela- tive to intersecting watercourses, railroad-, and bridges is usually added. After practice an officer rif ordinary aptitude can survey and map in this way nearly as much road as he can traverse at the assumed gait. Thirty miles have frequently been mapped in a single day with sufficient accuracy for the desired purposes. These maps contain inaccuracies which, in an ex- tended survey, may accumulate and become troublesome. Various means are resorted to for coordinating and combining the maps of adjacent territory which have 1 n made by different offi- During the Civil War the general maps of the Peninsula were compiled almost entirely from reconnoissances on horseback. The rail- road was accurately surveyed and served as a basis, while the wagon roads were mapped by different parties whose trails Erequentlj ci d other. In this wax it was practicable so to adjust the errors as to obtain a general map of considerable accuracy. In General Sherman's h to the ea the topographical parties, were kept a day in advance of the army. were enabled to furnish maps for the guidance id the commanding general and his subordinates t hroughout i he ',i tire march. In case iome1 hing mi i e than a road map is '. as for example a map of a po ible battlefield, much raphic detail should Own ">i the map. So far as practicable, everything which might have any material bear- -i a I. attic should be indicated For example, in addition to all mads suitable for tic emcni of troops, there should be depth "f cuts, position <d stone walls. ground relal ive dis- el of hill- and hollows, d rveyot can learn in a few I. tie able tu Mir- military map. Willi additional practice hi md judgment as to what hal .c, nit, d in a iey of the ■ ip and time fot ills. The usual i i of contours, which an- lin i, | i ; ,,.| 32 ENGINEERING INSTRUMENTS. points of equal height, is the best for the purpose and should be used wherever practicable. In order that a map may be of the great* I service, it is desirable that a number of copii - of it should be prepared and placed in the hands of the commanding general and his subordinates at the earliest moment after the completion of the survey. Several methods are in use for this purpose. One of the simplest is to trace the map •on tracing linen; from this, if daylight be avail- able, a number of prints can be made by the ordinary blue-print process. This work can be expedited by making from the tracing several brown-print negatives, which can in turn be used simultaneously if there be sufficient apparatus for making prints. If daylight be not available, print- may similarly be made by artificial light on bromide paper. Map duplication has also been effected with the aid of photography. A negative of the original map, usually on a smaller scale, is made with a camera. This nega- tive is then used for printing positives bj any of the usual photographic processes. For further detailed information on this subject, consult articles in Report on Proceedings of Interna- tional Congress of Engineers, Division of Mili- tary Engineering (Chicago, 1893) ; and Root, Military Topography and Sketching (Kansas City. lS'lfi). ENGINEERING INSTRUMENTS. The various instruments used by the engineer and surveyor in determining elevations, directions, and distances in their work of mapping land, and locating and laying out engineering works. They may be broadly divided into instruments for: (1) Measuring distances, (2) determining directions, (3) determining horizontal lines. (4i measuring angles, and (5) miscellaneous work. For measuring distances the instrument most commonly employed is the engineer's chain, which is usually either 50 feet or 100 feet long, and is composed of steel wire links alternately long and short, every tenth foot being marked by a special form of brass tag. At the two ends of the chain there are loop-shaped brass handles for convenience in use. Gunter's chain, invented by Edmund Gunter, an English astronomer, about 1020, is 66 feet long and is divided into 100 links, each 7.02 inches long. This chain is mostly used in land-surveying, where it is desired to obtain quickly distances in miles and areas in acres: for example, one mile equals 80 chains and one acre equals 160 square rods, equals ten square chains, equals 100.000 square links. This chain i- u-cil in all the United Slales land surveys; and in all deeds of land conveyance or other similar legal documents, when the word "chain' is used it. is Gunter's chain that, is meant. For city and bridge work, and accurate measurements gen- erally, steel tapes from three feet to 1000 feet, bill usually 50 feel ami 1(10 feet, in length, and graduated in meters and fractions or in feci, and inches, arc employed. The advantages of steel (apes OVei ill. mi .ire licit, they do not kink, stretch, or wear so as to change their length. lor measuring direction the most, simple in- t '"I"' ni employed i- the surveyor's oompass. This consists essentially of two uprights having vertical slils to give a line of sight, which are attached to a horizontal graduated circle at the re of w hich i- ii nted a magnet ic n lie tree to m. ne ii,,. whole being supported with de-