FISCHER: LENGTH STANDARDS AND MEASUREMENTS 155 



pared consisted of five separate end meters, each one of which 

 had been carefully compared with the committee meter, the 5 

 meters being placed end to end on a suitable support and kept in 

 contact by the tension of springs. This apparatus was used to 

 measure the Yolo base in California, the probable error of which 

 was computed to be 1 part in 1,750,000. 



The next bars used for this work were what are known as 5- 

 meter contact rods and consisted of a single bar of steel enclosed 

 in closely fitting wooden cases afterwards covered with padded 

 canvas. The temperatures of these bars were determined by 

 means of thermometers in contact with the bars some distance 

 from their ends which were read through windows in the wooden 

 case and convas covering. The method of using these bars in 

 the field was the regular method of mounting them on tripods 

 and placing them end to end. The accuracy of the measurements 

 of this style of bar in the hands of the party in charge of Mr. O. H. 

 Tittmann in 1891, was about 1 part in 1,700,000.- 



At the same time that this base was measured with bars 13 

 and 14, apparatus of an entirely new design by Dr. R. S. Wood- 

 ward was tried out on the same base. This apparatus, which 

 was described by Professor Woodward before this Society some 

 years ago, consists of a 5-meter bar which, in use, is carried 

 in a steel trough and covered with crushed ice, the trough in 

 turn being supported upon two trucks which travel on a portable 

 track. Posts are placed in the ground 5 meters apart and upon 

 them micrometer-microscopes are mounted. The operation of 

 measuring consists in bringing the bar under the first two micro- 

 scopes and then setting the cross wires of the microscope on the 

 lines of the bar. Then without disturbing the reading of the 

 forward microscope the bar is displaced longitudinally until 

 the line on the rear end of the bar is brought under the forward 

 microscope while at the same time an observer at the forward 

 end of the bar points on the line at that end, this process being 

 repeated until the base is completed. A kilometer measured in 

 this way is estimated by Dr. Woodward to have an accuracy of 

 about 1 part in 3,000,000, but the method is extremely expen- 



2 Appendix No. 8, U. S. Coast and Geodetic Survey Report for ISQllti' 



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