706 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



Chapter I. 

 PEIMAEY TRIANGULATIOK 



When the survey of Colorado was coinmeuced, in the spring of 1873, 

 by the United States Geological and Geographical Sarvey of the Terri- 

 tories, it was found necessary to inaugurate a system of primary tri- 

 angulation, in order to locate a number of points with a considerable 

 degree of accuracy, upon which the topographical work might be based. 



The first important step was to find a suitable location for the accu- 

 rate measurement of a base-line. This first step is all important, as the 

 future work depends entirely upon the accurate measurement of the 

 base. 



After reconnoitering the country in the. vicinity of Denver, Mr. Gard- 

 ner selected a spot just east of the city, where a '' tangent" of the Kan- 

 sas Pacific Eailroad could be used to considerable advantage upon 

 which to measure the greater portion of the base. Mr. Gardner has 

 kindly furnished me with the following details of the measurement : 



MEASUREMENT OF THE DENVER BASE. 



The base is a little over six miles long, and half of it is on- a '• tangent" 

 of the railroad. The west end of the base is 4,811,566 feet from the end 

 of the '- tangent" and on its western prolongation. 



Three and a half days were occupied in twice measuring this base. 

 The measurement was made with a Chesterman steel tape 100 feet long, 

 having a spring-balance attached, by which the tape was stretched, with 

 a tension of 16 pounds. The end of each 100 feet was marked with a 

 knife-edge on the track or on a low stool. The profile of the line was 

 leveled so that all inclined measurements might be reduced to horizon- 

 tal distances. The temperature of a mercurial thermometer exposed to 

 the sun was read every five minutes, and this was assumed to be the 

 temperature of the tape. After two measurements of the base, the steel 

 tape, without being used for further work, was taken to Washington and 

 compared with the United States Coast-Survey standard for chains. 



The following were the results of the two measurements of the Denver 

 base : — 



First measurement. 



Feet. 



Measured length corrected for temperature and slope 31861. 304 



Second measurement. 



Measured length corrected for temperature and slope 31863. 102 



Total correction applied for slope — 1. 924 



Total correction applied for temperature — 



To first measurement . , +2. 542 



To second measurement .. + 4. 67 



