B. 8. Woodward— Iced Bar Base Apparatus. 49 



Length of measuring bar. — The most important and diffi- 

 cult operation attending the use of the iced bar apparatus is 

 that of deriving the bar's length in terms of one of the Inter- 

 national Prototype Meters. As already stated, a preliminary 

 determination of this length was made in July, 1891, just 

 previous to shipping the apparatus to Holton Base, Indiana. 

 The method adopted in this and in most subsequent determi- 

 nations may be briefly described as follows : Six micrometer 

 microscopes were mounted in a straight line at intervals of one 

 meter. The 5 m distance between the two extreme microscopes 

 was measured with Prototype Meter, No. 21, and this distance 

 transferred from the extreme microscopes to the 5 m steel bar 

 No. 17. All comparisons of this kind were made with both 

 the meter and 5 m bars in melting ice. The 5 m bar was mounted 

 in its Y-trough and the meter bar in a wooden box. 40 to 45 

 kilograms of ice were used for the 5 m bar and 10 to 12 kilo- 

 grams for the meter. 



Assuming constancy of temperature of the bars under com- 

 parison the precision of the method just outlined evidently 

 depends on the stability of the microscopes used. The first 

 series of comparisons of July, 1891, and several series of 

 February to May, 1892, were made on the office comparator. 

 This comparator was not designed to meet the special require- 

 ments of the case and it did not meet them satisfactorily. 

 The microscopes were too unstable. Their instability was due 

 primarily to the manner of mounting them. They were each 

 suspended by a cast iron bracket from a wrought iron I-beam 

 about 6 m long which is supported at its ends on brick piers. 

 The focal planes of the microscopes fell about 0*5 m below the 

 beam. The beam was wrapped with cotton batting and cov- 

 ered with a wooden casing to prevent rapid temperature 

 changes. Resting as the beam does by friction on the piers, it 

 is in general in a state of longitudinal stress, which is fre- 

 quently relieved by vibrations communicated to the piers by 

 vehicles passing in the adjacent street. Changes in this stress 

 produce changes in the curvature of the beam and entail exag- 

 gerated motions in the microscopes. The temperature of the 

 comparing room (which is underground) changes very slowly 

 from day to day when not occupied long by observers or when 

 lighted for short periods. The comparisons in question, how- 

 ever, required occupying the room for some hours per day, 

 and the heat from the observers and the electric lights caused 

 notable changes in the temperature of the beam as well as of 

 the air in the room. These changes no doubt caused more or 

 less irregular displacements of the microscopes. 



Am. Jour. Sci. — Third Series, Vol. XLV, No. 265.— January, 1893. 

 4 



