WHEELER SECONDARY BASE IN GEODETIC SURVEYS. 3-5 



The Standard 300-foot steel tape and tape thermometer of the 

 Mississippi River Commission were transferred to the Missouri 

 River Commission, and the following improvements have ob- 

 tained in their use : 



(1.) Such preparations are made beforehand that a single measure- 

 ment of the base is made in the quickest time possible and 

 with the least change of temperature, that single measurements 

 at different mean temperatures maj be compared. 



(2.) The device of an apparatus called an adjuster, by which at the 

 moment the tape is recorded the tape is suspended at rest in 

 equilibrium under a known tension; this adjuster being in 

 parts, and being readily taken up, carried forward, and placed 

 again. 



(3.) The mark of measurement is made on a zinc strip, and transfer- 

 able to the office for a permanent record of the discrepancies 

 in measurements. 



(4.) The length of the tape is determined by measuring a known 

 base of primary triangulation, using the same adjuster and 

 method as is used on the secondary base. 



For a two-mile base about 500 stakes from 2 to 3 feet in length, 

 with a cross-section of 4 square inches, are required, and the time 

 for getting them out and setting them is about two days for a 

 party of ten men. A few men, with a transit and tape, set the 

 marking-stakes, place the zinc strips on them in line with the 

 direction of the base, and distribute the other stakes, a supporting 

 stake being at every even 30 feet of the tape. Others, with a 

 guide-frame, set the three platform-stakes about 2^ feet in front 

 of the marking-stakes, and the straining-stake for the rear end of 

 tape about 2^ feet back of the marking-stake. All are driven to 

 the required distance below the top of the marking-stake as shown 

 by the guide-frame, that the tape in use may rest on the zinc strip 

 without pressure. Others set the supporting-stakes just ofl'the line 

 and drive the nails for the friction-hooks. The heads of these nails 

 are in line with points 2 inches above the marking-stakes for any 

 one space between marking-stakes ; or, when the 2-inch double- 

 ended hooks are hung upon the nails, the lower ends of the hooks 

 are in line with the center of the top of the marking-stakes. 



Inasmuch as rapid work is desirable the tactics of a measure- 

 ment are important. 



