to build high towers if the distance be long and to overtop the trees if they interfere. In a perfectly level country, with a line of 20 miles, the towers must be 60 feet high to overcome the curvature of the earth and to make the line clear between the light and the theodolite. THE GREAT ARCS OF TRIANGULATION The progress of the Coast and Geodetic Survey is graphically shown in the map on page 664, which reveals the fact that a band of triangulation has been stretched from the frontier of Maine along the coast to the mouth of the Rio Grande. Here a line has been run due north to the Canadian border, where it intersects with the boundary between North Dakota and Minnesota. From this arc, at a point not far west of Fort Worth, a line swings westward to the California coast and thence to Puget Sound, and thence along the Canadian boundary to the line extending north from the mouth of the Rio Grande. Another great line cuts across the country from San Francisco due east to the Blue Ridge and Allegheny Mountains, where it intersects another line running from the Maryland line to Mobile, Alabama. This latter line has a greatly extended area in the vicinity of the approximate juncture of the boundaries of Tennessee, North Carolina, Alabama, and Georgia. A shortline has also been run across the northern neck of Florida, one up the Mississippi, one from a point. south of Denver to the Canadian line, and one from the North Dakota-South Dakota-Minnesota intersection to the western shore of Lake Superior. In addition to these, there are a number of disconnected surveys. here and there over the country and a number of reconnaissances or preliminary surveys. Upon these lines of triangulation are based all the exact knowledge we have concerning the exact location of surface of the United States as a geographic entity. THE METAL DISCS WITH WHICH THE COAST AND GEODETIC SURVEY MARKS ITS TRIANGULATION STATIONS Each triangulation is marked by brass and iron discs as shown in the picture. These marks are useful not only to members of the Survey who desire to relocate positions, but also to private surveyors who desire to connect their survey with one of these control points. Every survey must have a beginning, and the beginning point of the great national triangulation survey is a point in Kansas formerly known as the United States Datum, but now as the North American Datum. All surveyors' lines in the United States eventually will lead from here, as all roads led to Rome in a bygone civilization. Canada and Mexico have adopted the North American Datum, so that practically the whole of North America which has been connected trigonometrically is hitched to this point. So carefully have all the different surveys been made that they may all be put together and the lines fit perfectly. Any one familiar with surveys made on each side of a river, and how hard it is to get them to fit together, in ordinary practice, can understand what accuracy has been required to bring all the different surveys of North America into such har mony that from every point already fixed a skilled man from the Coast and Geodetic Survey could run his line right back to the North American Datum, in Kansas. Of late years many cities have found it necessary to order a triangulation of their territory, in order to connect their surveys with those of the Coast and Geodetic Survey. There was much confusion encountered in the offices of city surveyors in trying to connect their layouts of streets, lots, parks, and other works and areas with the big federal survey. These triangulations cover such cities as New York, St. Louis, and Boston, and are now in progress in San Francisco and Cincinnati. Maryland has had all her oyster beds located by the United States triangulation surveys and their positions fixed with reference to the United States Datum and the stars. DETERMINING THE EARTH'S SHAPE The work of the Survey in measuring with great accuracy the surface of the United States has not only had its practical uses, but its scientific uses as well. The diameter of the earth is the primary yardstick by which all celestial distances are measured, and the exact length of this astronomical yardstick cannot be known until the shape of the earth is determined with great accuracy. So important is this work that 21 nations are linked together in an international agreement for the support of the International Geodetic Association and the promotion of its work of earth-measurement. central bureau is maintained at Telegraphenberg, Prussia, and triennial conferences of the science of geodesy are held. The work of the Coast and Geodetic A ONE OF THE POWERFUL LIGHTS WHICH FLASH SIGNALS 100 MILES These signals are used to flash from one triangulation station to another making observations at night. The lamps are lighted by acetylene gas, and the reflectors are so powerful that the light has been seen through the telescope of a theodolite for a distance of over 100 miles (see page 665). Survey has proven so accurate that the world now recognizes the figure of the earth determined by it as the best available, and for that reason it was adopted as the standard by the International Convention of Astronomers, recently held at Paris. MAPPING WITH CAMERAS In recent years the Survey has been employing the camera in making its topographic surveys, thus being able in some instances to do away with the plane table. This method of topographic surveying is especially useful in such regions as Alaska, where difficult country and bad weather conspire against the successful use of the plane table. The cameras are specially prepared, cross wires being fixed to the lens and the scale being so arranged that they appear with the view upon the photographic plate. The chief precaution necessary is that each prominent topographic object shall be photographed from at least two points (see page 674). The work of triangulation and precise leveling sometimes requires the endurance of great hardships and affords many adventures. These involve all the dangers and experiences that go with scaling the Alps or climbing a Mt. McKinley. Tugging instruments up a mountain-side, where the slightest misstep would carry the bearer to the bottom of some terrible chasm or precipitate him into some madly rushing, ice-cold mountain stream, is no summer-evening picnic at best. Now one of the Survey engineers may be heading a party on the 141st meridian that divides Alaska from Canada, where today they scale a great mountain peak within the Arctic Circle, tomorrow battle OUR GUARDIANS ON THE DEEP with an epidemic of smallpox among the Eskimos, and a few days later are down in a little valley where the mosquitoes are so thick that even a horse cannot graze unless it be protected by nets. Now one of them is at work with a party in the Rocky Mountains, fording streams that are bordered with quicksands, shooting down rapids that threaten every moment to swamp canoe, party and all. But whatever the task of the men, they learn to live with the thought uppermost in their minds that he who is willing to bear the heaviest burden will find every other member of the party ready to help him bear it. Men go to Africa to hunt big game; they go to South America to explore new rivers; they go to Switzerland to climb high mountains; they go to the heart of Africa and the interior of Tibet to find strange people; but the men who make the basic surveys upon which our knowledge of the shape of the earth and our maps are based get all the thrills and all the experiences that come to them all, and get back to Washington safe and sound and ready for the prosaic work of writing up their notes. A MAGNETIC SURVEY Accurate surveying such as is involved in the triangulation work of the Coast and Geodetic Survey would not boot much on land or sea were definite information not at hand concerning the behavior of the magnetic needle. The magnetic needle refuses to stay put anywhere, and varies from the true north in different degrees in different parts of the earth. For instance, in London it changed its direction 35 degrees in the 232 years preceding the outbreak of the War of 1812. Then it began to swing back again, and up to the present time has moved east nearly 10 degrees. The change is by no means as rapid in the United States as it is in London, but at the same time it is enough to interfere with every survey if the rate of change were not known. As the rate varies between London and Washington, so it varies in different parts of the United States. In order to determine the declination of the magnetic needle innumerable ob 671 servations have been made all over the United States, and the exact declination for each place has been ascertamed. In the section east of the Rocky Mountain States there is scarcely an area 50 miles square where the variation of the needle has not been measured and passed on to the State and local surveyors, and in few places are these points of needle obser vation more than 20 or 30 miles apart. The method by which the variation at a given point is found is to compare the north of the compass needle with the true north revealed by the theodolite. The difference is the variation of the needle. sea. THE MAGNETIC NEEDLE IS NEVER STILL If the behavior of the needle is important on land, it is even more so on the A boundary a mile long fixed by the compass in Maryland in 1802 would now have its one end some 525 feet away from the original location if the survey now did not allow for the change in the variation of the compass since 1802. But on land points can be located without the use of the compass. On sea the compass is yet indispensable, and the navigator must constantly allow for its continual changes, else some fine night he would find himself high and dry on a reef when he thought he was miles away from it. For instance, the declination of the compass at Key West is 2 degrees east, while at New York it is 10 degrees west. Did a skipper steer through his voyage from Key West to New York on the assumption that the declination was the same at New York as at Key West, he would run into the coast somewhere south of a point 19 miles west of New York. The Survey maintains observatories at Cheltenham, Maryland; Baldwin, Kansas: Sitka, Alaska: Vieques, Porto Rico, and near Honolulu, Hawaii, where the behavior of the magnetic needle is under constant observation and study. There are places where the compass needle varies from forenoon to afternoon enough to make a difference of from 5 to 20 feet in a mile-long line, and places where there have been sudden and unaccountable changes such as occurred |