Page:The New International Encyclopædia 1st ed. v. 07.djvu/101

* ENGINEERING INSTRUMENTS. 8.1 ENGINEERING INSTRUMENTS. vices fur leveling. The use of the needle compass is con lined almost en i irely to land-surveying when extreme accuracy is not of great importance. It is used by selling up the instrument over our point of the line who,.' direction is to he deter- mined. .md 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 oil' tin' deviation of this line of sight from the north and south line indicated by the needle. The solar oompass has an attachment by means of which the line of sight is turned oil' from an image of the sun instead of from the magnetic meridian. For determining horizontal lines the engineer's h i 1 / 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 ami resting on a vertical pivot re- volving in a socket in the plate which fastens the instrument to its tripod or oilier support. Level- inn 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, Hie horizontal adjn of i he instrument being effected bj means of three or four (the latter number i the better) upright screws placed at equal distances between the plates, flic telescope is so fixed as to !»■ revers ible, and the adjustments arc in great part similar to those of other telescopic instruments, but are too numerous and minute lo be detailed here. Hot h horizontal plates being made, by means of the screws and levels, truly level, the telescope is pointed al one object, and the read ings-otl' from the graduated circle again per- formed; and by the difference of the readings the angular horizontal deviation is given, and when vertical angles arc required the readings are taken from a vertical circle in a similar manner. An instrument of tin- same construction, but the telescope of which cannot make a complete revolution on the horizontal axle, and thus does not 'transit,' is usually called a theodolite. The- odolites are commonly made larger and more pow- erful than transits, and arc mostly used in im- portant triangulation work for coast and geo- detic surveys, etc. The sextan/ (q.v.) i- a con en i< lit hand instrument for measuring angles universally and for making observations on ship- board, and also frequently used by engineer- in surveying when angles have to be measured from a boat, as in locating soundings, buoys, etc. The plane table consists of a suitably mounted draw- ing-board, on which rests a metal straight edge carrying an alidade supporting a telescope. By this instrument, instead of reading off certain horizontal angles, as is done with a transit, and then plotting them on paper, the directions of the various pointings are at once drawn on the paper, which is mounted on the drawing-board, no angles being read. Among miscellaneous engineer's instruments may be mentioned the aneroid barometer (q.v.) for determining altitudes; the pedometer for recording the number of stqps taken by a walk- ing man, which number multiplied by the length of step gives the distance traveled ; the odometer (q.v.), which records the number of revolutions of a wheel, which number, multiplied by the circum- ference of the wheel, gives the distance traveled; the clinometer, a device for measuring the slope of a hillside, etc.; the planimeter (q.v.), a device for measuring irregular areas which have been mapped to scale; the pantograph, for reproducing to the same or different scales from map- drawn on paper; and various drawing instruments, used in making maps and scale drawings of structures and machines. An instrument which deserves more particular mention is the heliotrope or heliostat, which is used by surveyors for render- ing distant stations distinctly visible. This is managed by placing a mirror at the distant sta- tion, and adjusting it so that at a particular hour of the day (arranged beforehand) the light of the sun shall be reflected from the mirror directly to the surveyor's station. The surveyor must make his observation almost at the instant he sees the glancing of the mirror, as the con- stant change in the sun's position in the heavens produces a corresponding change in the direction of the rays reflected by the mirror. Gauss (q.v.) invented such an instrument about 1821, which is used in geodetic surveying, and is said to possess such power that a mirror one inch square is visible eight miles off in average sunnv weather, and appears as a brilliant star at a dis-