ONE OF THE POWERFUL LIGHTS WHICH PLASH SIGNALS IOO MILES 



These signals are used to flash from one triangulation station to another making obser- 

 vations 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 avail- 

 able, and for that reason it was adopted 

 as the standard by the International Con- 

 vention of Astronomers, recently held at 

 Paris. 



MAPPING WITH CAMERAS 



In recent years the Survey has been 

 employing the camera in making its topo- 

 graphic 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 ar- 

 ranged that they appear with the view 

 upon the photographic plate. The chief 



precaution necessary is that each promi- 

 nent topographic object shall be photo- 

 graphed 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 ad- 

 ventures. These involve all the dangers 

 and experiences that go with scaling the 

 Alps or climbing a Mt. McKinley. Tug- 

 ging 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 



669 



