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 over- 

 come 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 bound- 

 ary between North Dakota and 

 Minnesota. From this arc, at a 

 point not far west of Fort Worth, 

 a line swings westward to the Cali- 

 fornia coast and thence to Puget 

 Sound, and thence along the Ca- 

 nadian boundary to the line extend- 

 ing 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 Alle- 

 gheny Mountains, where it inter- 

 sects another line running from the 

 Maryland line to Mobile, Alabama. 

 This latter line has a greatly ex- 

 tended area in the vicinity of the 

 approximate juncture of the bound- 

 aries of Tennessee, North Carolina, 

 Alabama, and Georgia. A short- 

 line 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 Da- 

 kota-South Dakota-Minnesota in- 

 tersection 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 pre- 

 liminary surveys. Upon these lines 

 of triangulation are based all the 

 exact knowledge we have concern- 

 ing the exact location of surface of 

 the United States as a geographic 

 entity. 



THE GREAT CASPAR SIGNAL, 



When surveying in Mendocino County, California, 

 a signal tower was built round a great tree, the trunk 

 of which was used to support the top of the tower, 

 and in this way the instruments were elevated to the 

 height of 135 feet above the ground. 



667 



