the American Metrological Society. 249 



The feasibility of this investigation, which has sometimes 

 been made the subject of doubts, expressed or implied, has 

 thus been fully demonstrated, and in the most effectual and 

 gratifying way. 



It may be added that this method of measurement, which 

 has been so successfully employed by Prof. Michelson in de- 

 fining the length of the meter, seems applicable to many other 

 physical researches, and will doubtless take its place among 

 the most accurate and important modes of measuring linear 

 dimensions, especially small ones. The processes by which it 

 has been possible to extend its application to the full length 

 of a meter-bar are very ingenious, and must reflect great 

 honor upon the gifted physicist who devised and executed 

 them. 



Other important metrological investigations have been car- 

 ried forward with energy during the year, among which I may 

 mention the extension of the hydrogen-scale for thermometers 

 to very low temperatures, where the alcohol-scale becomes un- 

 trustworthy. For practical use in determining temperatures 

 below —60° C, reason has been found to believe that the em- 

 ployment of toluene, or of ethylic alcohol, will be found ser 

 viceable. This question, as also that of the definite fixation 

 of the hundredth degree below 0° C, has received much at- 

 tention and the investigations are approaching definite results. 



It may be known to some of the society that a physicist in 

 the Netherlands, Mr. Bosscha, published some sharp criticisms 

 of the comparison of the recently established fundamental pro- 

 totype of the meter with the Metre des Archives the length of 

 which it had been intended to reproduce. The boldness of the 

 assertions was such as to cause some uneasiness among those who 

 were not especially conversant with the methods employed ; 

 although in one sense such an error as was alleged would only 

 possess a historic importance, since the new International Pro- 

 totype has been definitely adopted for the definition of the 

 meter. Yet inasmuch as it had been desired to reproduce the 

 length of the old Metre des Archives with all attainable pre- 

 cision, any appreciable deviation from this length would have 

 been a source of much regret. The elaborate comparisons 

 from which the assumed equality was deduced have been sub- 

 jected, during the last summer, to a new and very detailed re- 

 computation by the Director of the International Bureau, and 

 yield the same result as before. Mr. Cornu has investigated 

 the whole matter anew, and has discovered the source of the 

 discrepancies in erroneous methods employed by Mr. Bosscha, so 



