282 PROCEEDINGS OF THE AMERICAN ACADEMY 



found by M. Fizeau from a small section of the original bar wbich has 

 been preserved in the Archives. 



A third room of the building is devoted to the comparison of stand- 

 ards. This department is under the charge of Dr. Pernet. Here is 

 mounted a fine comparator by Brunuer Freres, built at a cost of 15,000 

 francs. A universal comparator by Starke, of Vienna, and costing 

 28,000 francs, will soon be in position. This apparatus will allow 

 comparisons between standards two meters in length. There are 

 also attachments for comparing subdivisions. 



The Bureau has also a very perfect apparatus for determining the 

 zero point and the boiling point of the thermometers employed in the 

 comparisons. The minimum point of freezing is first found. To this 

 minimum reading are applied the variations of the zero point, which 

 are investigated for each thermometer. The thermometers are made 

 by Baudin, of Paris. He makes two kinds. In one, the tubes are di- 

 vided into a scale of equal parts; in the other, all the errors, except 

 those of the zero and boiling points, are included in the graduations. 

 The thermometers are read to single hundredths of a degree. It is 

 the experience of Dr. Pernet that all the standard thermometers of 

 Baudin will agree inter se within three hundredths of a degree. 



I now pass to a consideration of the operations of the French Sec- 

 tion, which are conducted in the building of the " Boole des Arts et 

 Matiers," usually called the Conservatory. 



Through the kindness of M. Tresca, who is the secretary of the Sec- 

 tion, and, since the death of General Morin, the acting Director of the 

 Conservatory, I was able, during a recent visit to Paris, to make 

 a careful study of the entire operation of converting the '•Metre 

 des Archives " into an equivalent line-meter. Notwithstanding the 

 pressure of the official duties of M. Tresca, which were at the 

 time of my visit especially great, owing to the illness and death 

 of General Morin, the Director of the Conservatory, he gave me 

 several hours each day, explaining in detail each step of the opera- 

 tion, from the melting of the platinum-iridium to the final compari- 

 son of the completed bar with the " Metre des Archives." As the 

 result of this somewhat critical study, I express the unqualified opin- 

 ion that M. Tresca is entirely master of the problem. His methods 

 and his results are at least bej r ond present criticism. In this I do not, 

 however, include the method adopted of comparing end-measures with 

 line-measures. This method, which is sometimes credited to Fizeau, 

 and sometimes to Wild, seems to me to be radically defective. 



To the end of a plate having the same thickness as the " Metre 



