280 REPORT UNITED STATES GEOLOGICAL SURVEY. 



sas Pacific Eailroad could be used to considerable advantage, as the 

 greater portion of the base could be measured upon it. Mr. Gardner 

 has kindly furnished me with the following details of the measurement: 



MEASUREMENT OF THE DENVER BASE. 



The base is a little over 6 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 Ohesterman 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 

 ascertained by level so that all inclined measurements might be reduced 

 to horizontal 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 Wash- 

 ington and compared with the United States Coast-iSurvey standard for 

 chains. 



The following were the results of the two measurements of the Den- 

 ver base : 



First measurement. 



Feet. 



Measured length corrected for temperature and slope 31861. 304 



Second measurement. 



Measured length corrected for temperature and slope 318G3. 102 



Total correction applied for slope — 1. 924 



Total correction applied for temperature — 



To first measurement , +2. 542 



To second measurement + 4. 67 



Adopted measured length corrected for temperature and 



slope 31862. 203 



Correction for error in length of tape ..,...,... +5. 416 



Correction for reduction to sea-level — 7. 825 



Length of base-line - . 31859.794 



The uncertainty of this measurement cannot probably exceed one ten- 

 thousandth. Much of this error is doubtless due to the incorrect as- 

 sumption of the relations of the temx3erature of the tape to that of an 

 exposed thermometer. 



From this base the work was expanded by gradually increasing tri- 

 angles, with great care, to the high mountain peaks lying to the west of 

 Denver. 



The plan adopted in this work was to use the highest and more promi- 

 nent peaks as stations, taking great care to select such points as would 

 give a good system of well conditioned triangles, and at the same time, 

 where possible, selecting the sharper and most definite points, as they 

 could be sighted more accurately, especially at long distances, when the 

 monuments were not visible. Where the points were not sharp or well 

 defined it was generally found best to sight some object, such as large 

 rocks, or, on low points, trees, whose distance from the point where the 

 station was afterward made could be measured. Always on occupy- 

 ing a point there was built a large stone monument, which could be 



