84 B. S. Woodward — Iced Bar Base Apparatus. 



practicable the efficiency of the various forms of base appara- 

 tus used by the Survey and especially the efficiency of long 

 steel tapes or wires. Accordingly, considerable study was 

 given to this subject during the autumn of 1S90 and the 

 winter of 1890-1, and the plans and specifications for the iced 

 bar apparatus considered in this paper were matured and 

 approved early in the spring of 1891. It was constructed in 

 Washington, partly by the machinists E. K. Gray and Co., 

 and D. Ballauf, and partly by the Instrument Division of the 

 Survey. 



Before proceeding to a description of the apparatus I desire 

 to acknowledge my indebtedness to colleagues of the Survey 

 for valuable suggestions and criticism. I am specially in- 

 debted to Mr. John S. Siebert, who verified all of the pre- 

 liminary calculations relative to the stability and efficiency of 

 the apparatus, and who elaborated many of the designs and 

 made most of the working drawings for its construction. I 

 am particularly indebted also to Mr. E. G. Fischer, chief 

 mechanician of the Survey, whose knowledge of and skill in 

 mechanical appliances were frequently appealed to. Finally 

 it affords me pleasure to state that my friend Mr. E. S. 

 Wheeler, who has had extensive experience with base appara- 

 tus, happened to visit Washington about the time the plans for 

 this apparatus were completed and gave me the benefit of his 

 advice and criticism. 



Description of Apparatus. 



The measuring bar. — The measuring bar of this apparatus 

 is a rectangular bar of tire steel. It was rolled in the steel 

 works at Lancaster, Pa. It is 5'02 m long, 8 mm thick and 32 mm 

 deep. A cross-section is shown at A in the accompanying 

 drawing. 



The upper half of the bar is cut away for about 2 cm at either 

 end to receive the graduation, plugs of platinum-iridium, 

 which are inserted so that their upper surfaces lie in the neu- 

 tral surface of the bar. Three lines are ruled on each of these 

 plugs, two in the direction of and one transverse to the length 

 of the bar. These lines were ruled by Mr. Louis A. Fischer, 

 Adjuster in the Weights and Measures Office. The longitudi- 

 nal lines, which serve to limit the parts of the transverse lines 

 used, are 02 mm apart. 



To secure alignment of the bar eleven German silver plugs 

 of 5 mm diameter are inserted at intervals of 195 mm along the 

 bar so that they project about l mm above its top surface. The 

 upper surfaces of these plugs are all the same distance, within 

 a few hundredths of a millimeter, from the neutral surface of 



