WILSON ON GEOGRAPHICAL WORK OF THE SURVEY. 707 



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 temperature 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 could be measured from 

 the point where the station was afterward made. Always on occupy- 

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

 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 triaugulation had been extended into Southwestern Colo- 

 rado, a second base or base of verification was measured in the San Luis 

 Valley and connected with the large triangles of the principal system 

 by a smaller scheme. 



The base is about 5^ 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, a distance of about 5^ miles. 



The position was selected by James T. Gardner, a?id the measure- 

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

 fessor Atkinson, and Clarence Kelsey. 



The total time occupied in measuring the base twice was sis 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 wereof 2ineh plank aiul were 1 foot square. Each 

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

 12 B H 



