708 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



were placed on the line 100 feet apart. The Chestermau tape was then 

 stretched from the initial point on the first to the second stool, being 

 pulled straight by a strain of 16 pounds, applied with a spring-balance. 

 The 100 feet was then marked on the top of the stool with a pencil-edge. 

 The tape was then stretched jfrom this mark to the third stool. When 

 this 100 feet had been marked as before, the first stool was brought 

 forward. Each time that the tape was stretched, the bulb of a sensitive 

 thermometer was placed against the under side of the tape and its tem- 

 perature recorded. The difference of level between the stools was ascer- 

 tained with a leveling-instrument and rod. 



The temperature of the tape in the sun, when the breeze was light, 

 was found to be 5° to 6° higher than that of a thermometer exposed to 

 the sun. The results of the two measurements of the base are as 

 follows : 



First measurement. 



Feet. 



Uncorrected measurement 28534. 87 



Corrections for slope ....,.., — 10. 67 



Corrections for temperature -f 1. 40 



Corrections for error of tape from United States standard 



at62o , -2.86 



Corrected length of base , . . = 28522. 74 



Second measurement. 



Uncorrected measurement 28533. 895 



Correction for slope —10. 67 



Correction for temperature + 2. 193 



Correction for error of tape from United States standard 



at 620 ,. — 2. 86 



Corrected length of base 28522. 558 



Difference of two measurements 0. 18 



Adopted mean 28522. 65 



Correction to sea-level —10. 89 



Length of base 28511. 76 



These two measurements of this line, 5J miles long, differ only about 

 2 inches, which is a much more accurate result than that obtained at 

 the Denver base, where the tape was laid on the railroad track and the 

 temperature of a thermometer exposed to the sun was assumed to be 

 the same as that of the tape. San Luis base is of course entitled to 

 much more weight than the Denver base in the final adjustment of the 

 triangulation. 



AZIMUTH OF SAN LUIS BASE. 



For determining the azimuth of the base, six observations were made 

 on Polaris at the north end of the base, September 5, and four observa- 

 tions, September 6, at the south end of the basej six observations had 



