coated with hot asphalt, and over the top of the post is riveted a bronze tablet similar to that first described. These posts are set in the earth with only one foot of their length projecting above the surface, and wherever possible a flat stone is placed at the bottom as a suitable rest.

In order that these bench marks should be so marked as to fulfill the apparent intention of Congress and supply the popular demand by showing approximate elevations above mean sea level, it was decided that they should not be given ordinary serial numbers, but that the approximate elevation to the nearest foot should be stamped upon them, and these figures should constitute their numbering. It was soon found, however, that the assumed elevation above sea level as determined for a given point might be changed during a season or within a few seasons, and that this might change by several feet the relative elevations stamped upon two bench marks adjacent one to the other. To satisfy both conditions, therefore--the popular demand for sea-level heights and the scientific demand for exact differences between adjacent bench marks the following method was adopted:

The elevation above mean sea level to the nearest foot, as determined at the time of running the levels, is stamped upon the bench mark, and, in addition, an abbreviation of the name of the central datum point. Thus, for an elevation depending on North Creek, the mark would be, say, "1728 N. C." on tablet or iron post. This datum being subsequently found to be in error about 5 feet, later bench marks established in that neighborhood are marked as though resting upon a different datum, that of Albany, from which the check was obtained, and the next bench mark to that above cited may be stamped "1810 A." An engineer running between these two and finding them out according to his levels by 5 feet, will at once realize that the letters "N. C.” or the letter "A." carries some special meaning, and will doubtless write to the office of the Geological Survey to inquire concerning it, when the desired information will be furnished.

The primary leveling is done by first-class methods, but not of such quality as would classify it as precise leveling. In order to check and eliminate the errors incident to such work, lines of levels run within an area under topographic survey are so arranged as to close back on themselves in circuits or polygonal figures, these lines being run by a single levelman and rodman; the closure of various circuits, then, leads at once to the detection of any gross error. Unusually long or important lines are run by a levelman and two rodmen in such manner as to make a duplicate rodded line.

In order to establish as nearly uniform grade of accuracy for the work as practicable, general instructions have been issued to the levelmen, and are printed in the "Instructions governing the topographic branch." The allowable limit for closure error for good work is set

at 0.05✓D feet, in which D represents the length of the line or circuit in miles.

The instruments in general use are 20-inch engineer's levels and New York rods, the latter furnished with plumbing levels and with a specially constructed foot plate. The only respect in which the rods differ from those regularly furnished consists in making the foot plate a truncated pyramid of brass, the bottom of which is but one-half inch in width. This is in order to keep it freer of dirt and to compel its being placed more accurately on the top of the steel turning peg. The only change from the regular form made in the instrument is in increasing the sensitiveness of the level bubble so that 1 division on the tube equals 10 seconds of arc. All turning points must be on steel pegs at least 12 inches in length and driven firmly into the ground.

In addition to the ordinary primary and flying levels, the Geological Survey has run several lines of precise levels, most of which form elements in the precise-level net extending from the Atlantic coast to the Mississippi River and the Gulf of Mexico, which comprises in addition the work of the Corps of Engineers of the Army, the Coast and Geodetic Survey, the Lake Survey, and the Pennsylvania and the Baltimore and Ohio railroad companies.

The precise-leveling instruments used by this Survey differ from the ordinary spirit levels in many particulars, the effect of which is to make them high-grade instruments of precision, 8-inch and 4-inch bubble tubes being used. The rods are of special design, 10 feet long, double-faced target reading and nonextensible, and are made from selected white pine and treated with boiling paraffin or linseed oil. The lines are run with two rods and rodmen, an umbrellaman, and a laborer. Instructions for the conduct of the work differ from those for primary work in the following essential points:

The instrument is at all times shaded from the wind and the sun; fore and back sights are of exactly equal lengths, and no sight over 300 feet in length is permitted except in special cases. The instrument is unclamped from the tripod after the legs have been firmly set. Four steel turning points are used at the same time, two for each rodman, and they are so used that those employed for backsights are not withdrawn until the foresight pegs have been set; thus the line depends at no time on the stability of the instrument, as there are always two turning points in the ground. The limit of apparent error in the work is fixed at .03 distance in miles, which represents the allowable difference, in feet, in the elevations of any bench mark as determined by the duplicate lines.

Office work. The office work of triangulation consists in reduction to center, station adjustment, computation of spherical excess of triangles, and adjustment of figures by the method of least squares; also the computation of distances and azimuths between and the geo

detic coordinates of from 300 to 400 primary and secondary points per annum. The positions of all points in the Appalachian region from Maine to Alabama, those along the thirty-ninth parallel from the Atlantic to the Pacific, and those along the ninety-eighth meridian are now computed upon the United States standard datum.

The office work of primary traverse consists in the reduction of all azimuth observations and the adjustment of the line between azimuth stations, and of the computation of between 7,000 and 8,000 latitudes and departures and of 1,000 geographic positions per annum.

The results of primary triangulation and traverse are summarized and published at the close of each fiscal year in the form of a bulletin (Series F).

The office work of level adjustment consists in the preparation of plates showing groups of level circuits by States, with an indication of the error of each circuit. Where a group of primary level lines. connects with the precise-level net at several points the errors are corrected by distribution in proportion to mileage, after determining weighted values for the intermediate points; but a group of circuits. extending from one point or circuit is treated by adjusting first the circuit in which the largest closure (within limits) occurs and then correcting the remaining closure errors of successive circuits in proportion to the mileage. This method answers all practical purposes. The vast amount of leveling data sent in from all parts of the United States by the numerous field parties would not admit of a treatment by the method of least squares. As showing the amount of this class of work, it may be stated that during the last fiscal year (1902–3), 29,160 miles of levels were adjusted, correcting the elevations of 1,826 bench marks.

The results are annually summarized and published in the form of a bulletin (Series F).

Pl. VI shows the progress made in astronomic location and primary control to January 1, 1904.

For publications on this subject see the Survey's list of publications.

Section of Inspection of Surveying and Mapping.

Coincident with the reorganization of the topographic branch in 1903 provision was made for the inspection of topographic surveying and mapping, to be carried on through consultation and collaboration with the section chiefs. To the chief of this new section was also given the direction of the preparation of such base maps as might be required. for various purposes. The inspection of field work and the final drawing of the maps lie entirely outside of the duties of administration, are general in character, and consist chiefly in keeping in touch with the various topographers, criticising their expression of topographic