July 7, 1916] 



SCIENCE 



urement of the base lines for controlling the 

 lengths in triangulation. In this work the 

 Survey has furnished much of interest and 

 of value to the geodesist. Ever has it kept 

 keenly before it the necessity for refined 

 measurements, and many valuable devices to 

 accomplish this desired result have been 

 added by members of the force. 



BASE BARS 



The Duplex bars, invented by Assistant 

 William Eimbeck, are probably the best 

 form of base bars ever devised and gave a 

 very high degree of precision. But they 

 were soon replaced by the tape as a form of 

 base apparatus. 



The only bar used in the United States, 

 and probably in the world, which gives en- 

 tire satisfaction, so far as accuracy is con- 

 cerned, is the iced bar, designed by Presi- 

 dent R. S. "Woodward of the Carnegie Insti- 

 tution when an assistant in the Survey. 

 Owing to the great cost per kilometer of 

 base of using this form of apparatus for 

 field work, when compared with the cost of 

 using tapes, the iced bar is now used only 

 for standardizing other apparatus, and for 

 this purpose it remains unexcelled. 



STEEL TAPES 



In the Coast and Geodetic Survey the 

 tape has supplanted the other forms of base 

 apparatus. Credit for the introduction of 

 steel wires and tapes for this purpose must 

 be given to Professor Jaderm of Sweden, 

 but it was the accurate and extensive in- 

 vestigations made by Assistant Woodward 

 in 1891 which caused the adoption of tapes 

 by the Survey. He proved that steel tapes, 

 when used at night, and standardized under 

 the same conditions that prevail during the 

 base measures, gave essentially the same 

 high degree of accuracy as the Duplex 

 bars, with about one third of the cost and 

 with far greater rapidity. It is practically 



certain that no more base lines will be 

 measured by base bars, at least in the 

 United States, except when it is necessary 

 to standardize the tapes. 



The remarkable measurement of nine 

 base lines in one season, in 1900, by a single 

 party constitutes a noteworthy achieve- 

 ment. The nine bases had a total length of 

 43 miles and furnished a control of over 

 1,000 miles of triangulation. In order to 

 eliminate constant errors five different sets 

 of apparatus were used, and an average 

 accuracy corresponding to a probable error 

 of -1 part in 1,200,000 was secured. With 

 this work a new epoch in base line measure- 

 ment was introduced, for it proved, through 

 the most rigid of tests, that the tape had 

 no superior for speed, economy and ease of 

 manipulation. 



INVAE TAPES 



In the use of invar tapes, base meas- 

 uring took another long step forward. 

 Many severe tests have fully proved their 

 excellence. They are found to possess prac- 

 tically all of the good features of the steel 

 tapes, but have the added advantage that 

 they enable bases to be measured in the 

 daytime and even in the sunny days, a fact 

 due to the small coefficient of expansion of 

 invar, which is only about one thirtieth 

 that of the steel tapes. 



Recently the plan has been adopted of 

 having the bases measured by the triangu- 

 lation party. By it base measurement has 

 become simply an incident to the triangu- 

 lation, and the cost has been reduced to 

 about $60 per kilometer, a sum which is in 

 great contrast to about $300 per kilometer 

 with the Duplex bars. 



PRECISE LEVELING 



Practically all of the great nations of the 

 earth have been actively engaged upon the 

 difficult problem of determining the cor- 



