An instrument of the 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 thcodotite. The odolites are commonly made larger and more pow erful than transits, and are mostly used in im portant triangulation work fur coast and geo detic surveys, etc. The sextant (q.v.) is a con venient hand instrument for measuring angles universally and for making observations on ship board. and also frequently used by engineers 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. thy 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 stops 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 maps 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 sim'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 1321, which is used in geodetic surveying, and is said to possess such power that a mirror one inch square is eight miles off in average sunny weather, and appears as a brilliant star at a dis tance of two miles; while some heliotropes have been used so powerful as to be visible nearly 200 miles off. The term heliostat, applied by Captain Ibn:wmond to an instrument invented by him for the satne purpose, more properly belongs to an instrument invented by 'sGravesanile, consisting an equatorial revolving on its polar axis, so that the sun. when once accurately in the focus of the telescope. continues steadily fixed there. Drionnumd's heliostat is chiefly used in Great Brim in.
Fe r full discussions and descriptions consult: Stanley, ,eurreying and Leveling losfruntents (London. 1891) ; Baker. Engineer's Nur•eying bist ru ts ( New York, 1892). See SURVEYING.