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 instniments for: (I) Measuring distances, (2) determining directions, (3) determining horizontal lines. (4) measuring angles, and (5) miscellaneous work.
For measuring distances the instrument most commonly employed is the engineer's chitin, 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 1620, is 66 feet long and is divided into 100 links, each 7.92 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 SO chains and one acre equals 160 square rods. equals ten square chains, equals 100,000 square links. This chain is used in all the United States land surveys; and in all deeds of land conveyance or other similar legal documents. when the word 'chain' is used it is Cunter's chain that is meant. For city and bridge work, and accurate measurements gen erally, steel tapes from three feet to 1000 feet, but usually 50 feet and 100 feet in length, and graduated in meters and fractions or in feet and inches, are employed. The advantages of steel tapes over chains are that they do not kink, stretch, or wear so as to change their length.
For direction the most simple in strument employed is the svrveyor's compass. This eonsists essentially of two uprights having vertical slits to give a line of sight, which are attached to a horizontal graduated circle at the centre of which is Ill oun ed a magnetic needle free to move, the whole being supported with de vices for leveling. The use of the needle compass is confined almost entirely to land-surveying when extreme accuracy is not id great importance. It is used by setting up the instrument over one point of the line Nvlinse direction is to he deter mined.: nd sighting through the upright slits at a mark or rod set at another point of the line, and then by means of a graduated plate reading off the deviation of this line of sight from the north and south line indicated by the' needle. The solar compass has an attachment by means of which the line of sight is turned off from an image of the sun instead of from the magnetic For determining horizontal lines the engineer's level is the instrument most commonly employed. This instrument consists of a telescope clamped in Y-shaped uprights rising from a bar carrying a spirit-level and resting on a vertical pivot re volving in a socket in the plate which fastens the instrument to its tripod or other support. Level ing-screws permit the adjustment of the spirit level bar to a horizontal position, and, as the axis to the telescope is parallel to this bar, the line of sight is horizontal. In connection with the
level there is used a. level rod graduated to feet and fractions of feet, and having a target. which can be slid up and down the rod to coincide with the line of sight through the telescope. The form of level just. described is often called a Y level. A dumpy level has a short telescope with a wide aperture. An architect's level has a com pass attachment. The level is used to find the relative elevation of points a considerable dis tance apart, to obtain the profile of a line, and to establish a grade. See LEVELING.
For measuring angles the instrument most commonly used by engineers is the transit. This is the most useful and universal of surveying in struments. Besides measuring horizontal and vertical angles it will read distances by means of stadia-wires. determine bearings by means of the magnetic needle, do leveling by means of a bubble-glass attached to the telescope, and do the work of a solar compass by means of a special device attached for the purpose. The transit con sists essentially of two concentric circular plates of copper, brass, or other material (the upper plate or upper horizontal either being smaller, and let into the lower, or lower horizontal, or the rim of the lower raised round the outside of the upper) moving round a common axis, which, being double, admits of one plate moving inde pendently of the other. Upon the upper hori zontal rise two supports, bearing a cross-bar, which is the axis of a vertical circle moving in a plane at right angles to the former. This latter circle either has a telescope fixed concentric with itself, or a semicircle is substituted for the circle, and the telescope is laid above and parallel to its diameter. The circles, as their names de note, are employed in the measurement of hori zontal and vertical angles. For these purposes the outer of the horizontal circles is graduated, and the inner carries the index-point and the verniers (q.v.) ; the vertical circle is also grad uated, and the graduations are generally read off by an index-point and vernier firmly attached to the supports. The upper horizontal is furnished with two levels placed at right angles to each other for purposes of adjustment, and has a compass box let into it at its centre. The stand consists of a circular plate supported on three legs and con nected with the lower horizontal by means of a ball-and-socket joint, the horizontal adjustment of the instrument being effected by means of three or four (the latter number is the better) upright screws placed at eqintl distances between the plates. The telescope is so fixed as to he revers ible, and the adjustments are in great part similar to those of other telescopic instruments, but are too numerous; and minute to be detailed here. Both horizontal plates being made, by means of the screws and levels, truly level, the telescope is 'Minted at one object. and the read ings-off from the graduated eirele again per formed; and by the difference of the readings the horizontal deviation is given, and when vertical angles arc required the readings are taken from a vertical circle in a similar manner.