22 TRANS. ST. LOUIS ACAD. SCIENCE. 



The secondary base apparatus, latest in use in this country^ 

 and therefore presumably the best, is of two forms, each based 

 on primary apparatus. The one — that of the Lake Survey of 

 1871, described in Report for 1872 — is a very approximate copy 

 of the Bache-Wiirdeman apparatus; the other — that of the Coast 

 and Geodetic Survey, described in Appendix 17, Report for 1880 

 — has the principle of the Repsold apparatus in determining the 

 temperature through the unequal rate of expansion in zinc and 

 steel. Each has end-contact apparatus and are arranged for use 

 on tripods. There is a very uncertain amount of error through 

 backward pressure depending on the stability of the tripods. 

 Even in the delicate primary Bache-Wiirdeman apparatus on the 

 Keweenaw base, this formerly insignificant source of error was 

 corrected by applying a correction of 0.66 of an inch.* 



With the Lake Survey secondary apparatus the measurements 

 were preferably made on a railroad track, the tubes resting on 

 the rail and supported in position by hand, and, when thus made, 

 the measurement must be transferred to stations alongside. The 

 passing trains disturbed the accuracy of such measurement. 

 Formerly if a degree of accuracy of 1 in 200,000 was obtained , 

 the work was considered excellent. 



Under the Mississippi River Commission the Assistant Engi« 

 neer had made use of the Lake Survey apparatus, and also of a 

 300-foot steel tape. They had introduced many refinements in 

 the use of the tape, such as the suspending of it on wire hooks, 

 to allay friction and for better alignment, and in giving a known 

 tension by means of a known weight. 



They had also determined the co-efficients of expansion and 

 elasticity for this tape. But the results for measurements were 

 uniformly rejected in favor of measurements by the secondary 

 base apparatus. The assistants, however, had great confidence 

 in the tape measurements could they be made by night or on an 

 overcast day. 



Accordingly on the Missouri River Triangulation, at the out- 

 set, it was decided to use the steel tape exclusively, and make 

 the measurements by night. 



* See § 37, chap, iii., of No. 24, Professional Papers, Corps of Engineers U. S. A. 



