SURVEYING INSTRUMENTS. The in struments used in the business of surveying fall into two general classes: those used in the held work and those used in recording the field work on maps. In both classes there are three objects to be attained, and suitable instruments are provided for each — the marking of points, the measurement of lines and the measurement of angles.
In field work the aping is the instrument used for the temporary location of a point. It is made of stout iron wire, or slender steel wire, about eight inches long, and with a ring turned at the top. Ten pins go in a 'set.* In field use it is customary to tie a bit of red flannel rag in the ring of the pin, so that it may be found quickly by the chainman. If the point is one which is to be marked permanently, the pin is replaced by a wooden with a copper nail, or with a stone monument marked with a simple 'cross cut into the stone, or a bronze bolt May be set into a hole drilled in the top of the monument.
For making linear measurements a long-rec ognized standard has been the surveyor's or Gunter's chain. It is 66 feet in length, made up of 100 links of stiff iron wire or steel, with a ring-shaped loop at each end, the adjacent links being connected 'by two rings. Each measured link is 7.92 inches long, including the span of a connecting• ring at each end. This chain is still in use for retracing the lines of old sur veys in which it was originally used, particularly in the resurvey of government public lands. The more modern engineer's chain is 100 feet long, and has 100 links; each, with its two ad jacent rings, comprising one foot. Both of these types of chains are comparatively rude in struments, and for close work have been sup planted by the narrow steel ribbon or tape, which, however, in surveyors' parlance goes by the name of This is a continuous strip of steel, one-eighth to three-sixteenths of an inch in width and one-fortieth to one-thir tieth of an inch in thickness. It may be 50 feet or 100 feet in length. Ten-foot divisions are marked by small blocks of brass soldered upon the steel, the exact points being indicated by notches filed in the brass. These ribbons are generally provided with a compensating handle for the forward end of the ((chain.° This Ilan. dle.is of two sections of brass tubing, one of which contains a thermometer and an adjust able scale, so that the length of the echains may be made longer or shorter according to the reading of the thermometer—the hav ing been tested according to a given temper ature. The other section of the handle contains a spirit level, to ensure that the measurement is strictly horizontal; and a stout helical spring which must be drawn out to an index mark, and thus ensures that in every measurement the same degree of tension is exerted upon the This type of chain cannot be bundled up as with the link type, and a folding reel is provided upon which it is safely carried about. For short and precise measurements the steel tape line is employed. This is a ribbon of steel about half an inch in width and one-eightieth to one-hundredth of an inch in thickness. It is graduated in feet, tenths and hundredths of a foot, so etched upon the steel that the divisions appear in relief. This type of tape line rolls up by means of a cranked handle into a leather case. Linen tapes and combination tapes of
linen and brass wire are useful in some in stances, but cannot be depended upon where ac curacy is important. Tapes and ribbon °chains)" may be authoritatively tested if sent to the Su perintendent of the United States Coast Survey at Washington. Measurements of extreme accuracy, as in the establishment of a base line for triangulation, are made with "base bars" or °compensation bars" so constructed of iron and brass that they remain the same length regard less of changes in temperature. These bars are carried about in cases of wood, and are used in sets of six, being each supported on two stout tripods. Another form of base bar is provided with a trough in which is kept continually while in use a mixture of ice-water and ice — thus ensuring a constant temperature of 32°, to which the bars are standardized. The steel alloy known as (q.v.) is also used with entire satisfaction within a certain range of temper atures, in which the metal does not expand in length. For measuring small vertical distances, as in leveling, a telescoping rod is used. The type most in use is known as the New York Leveling Rod. This rod is of maple, about two inches square and nearly seven feet in length, and has a brass shoe and a brass cap. The face of the rod is graduated into feet, tenths and hundredths of a foot, up to 6.50 feet. A slid ing target of oval outline has its centre cut away so that the graduations cif the rod are visible. On one of the inner edges of the tar get a vernier is provided so that thousandths of a foot may be read. The rod is made in two longitudinal sections dovetailed together, the back section sliding upon the front. When a vertical distance of more than 6.50 feet is to be measured, the target is clamped at that fig ure, a lower clamp loosened and the back part of the rod raised upward until the target comes into line. The? reading now is taken on the side of the rod,'which also is graduated, so that distances up to 12 feet may be measured to the thousandth of a foot. To ensure the verticality of the rod in the hands of the rodman, when meas urements are being made, the target is some thnes constructed with two target faces, one three or four inches back of the other, the ends of the front face being cut away, so that a yievr of both discs is had at once. Any deviation from a vertical position is immediately noticed through the lack of agreement of the horizontal line of both discs. There are several other forms of leveling rods, but in the United States the New York rod is almost universally in use. °Speaking rods" are those which are so painted as to be easily read by the observer • at the in strument, without dependence upon the rodman. This type is exceptionally useful in rapid re connaissance work, where long sights are taken. They are not so accurate, however, as the tar. get rod when it is carefully used. The type most in favor is the Philadelphia rod, in which the graduation is in figures six one-hundredths of an inch in height, set with their centres on the lines marking the tenths of the foot. It is quite easy for the leveler to estimate the hun dredths of a foot with great accuracy, but im possible to read to thousandths.