R. S. Woodward — Iced Bar Base Apparatus. 53 



The time required for the bar to acquire a sensibly stable 

 length is less than ten minutes. The rate of temperature- 

 change is so great in the early stages of freezing that ninety 

 per cent or more of the contraction of the bar occurs within a 

 minute after it is well surrounded by ice. The corresponding 

 time required by the Prototype meter to reach a stable length 

 appears to be less than five minutes, which is less than the 

 time essential to properly pack it in ice. 



A query may also arise as to whether the bar, resting as it 

 does with considerable friction on the vertical adjusting screws 

 of the Y-trough, may not change length by reason of longi- 

 tudinal stress communicated by the trough. In answer to this 

 querj- it may be said that the experiment of putting the 

 trough alternately under tension and compression in quick 

 succession has been tried on several occasions without disclos- 

 ing any effect on the bar. 



With regard to the precision attainable in the measurement 

 of a line with this apparatus, it would appear that the error of 

 operation may be rendered insignificant in the mean of a few 

 measures of a line, so that the probable error of the mean 

 length may be diminished to that of the bar when expressed 

 as a fraction of its length. It appears practicable to de- 

 termine that length with a probable error not exceeding 

 l/5000000th part, and this, therefore, would appear to be an 

 attainable precision in the measurement of a line with the 

 apparatus. 



Although the use of this or similar apparatus is not to be 

 recommended for primary bases in general, since it gives a 

 needless precision, it will compare favorably I think on the 

 score of economy with any of the earlier forms of apparatus 

 which have given a precision approaching the millionth part of 

 a measured line. The proper function, however, of the iced 

 bar apparatus appears to be that of an intermediary between 

 the lighter and cheaper forms of base apparatus and the stan- 

 dard meter. By means of this bar it appears practicable to 

 standardize long steel tapes so accurately that they will give all 

 needful precision for bases in general at much less cost than 

 other forms of apparatus.* 



Office Coast and Geodetic Survey, Sept. 15, 1892. 



* Experience of the author in the use of 100 m steel tapes with mercurial ther- 

 mometers to give temperature, on the Holton Base, 1891, shows that the length of 

 such a tape can be determined with a probable error not exceeding l/200U000th 

 part, and that the probable error due to errors of operation and temperature in the 

 mean of n measures of a line k kilometers lona: need not exceed 



±2 r 



VI 



