wiLsox.J MEASUKEMENT OF THE SAN LUIS BASE. 281 



sighted afterward with ease at a distance of from 30 to 40 miles. In 

 nearly all cases these monuments are the points given as the stations; 

 in many cases the monuments were built before the point was occupied, 

 and in such case the observations were reduced to center of the monu- 

 ment. 



THE SAN LUIS BASE. 



When the triangulation had been extended into Southwestern Colo- ^ 

 rado, a second base or base of veriiication was measured in the iSan Luis 

 Valley and connected with the large triangles of the principal system 

 by a smaller scheme. 



The base is about 5J miles in length. It begins on the eastern ex- 

 tremity of a low gravelly ridge on the north side of Kerber Creek, near 

 the stage-road crossing, and stretches northward diagonally across the 

 valley. 



The position was selected by James T. Gardner, and the measure- 

 ment conducted by him with the assistance of Eobert Adams, jr., Pro- 

 fessor Atkinson, and Clarence Kelsey. 



The total time occupied in measuring the base twice was six days, 

 commencing August 24, 1874. Flags were placed along the line at such , 

 short intervals that three were always in sight, from which the tape 

 could be aligned by the eye. Low stools were used for marking the dis- 

 tances. Their tops were of 2-inch plank and were 1 foot square. Each 

 was supported by four iron spikes 6 inches long. Three of these stools 

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

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

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

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

 The tape was then stretched from 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 sensi- 

 tive thermometer was placed against the under side of the tape and its 

 temperature recorded. The diif'ereuce of level between the stools was 

 ascertained with a leveling-iustrument and rod. 



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

 was found to be 5° to 0° higher than that of a thermometer exposed to 

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

 lows : 



First measurement. 



Feet. 



Uncorrected measurement = ^ 28534. 87 



Corrections for slope — 10. G7 



Corrections for temperature - + 1. 40 



Corrections for error of tape from United States standard 



at 620 — 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 



