PHYSICS. 



By George F. Barker, 

 Professor of Physics in the University of Pennsylvania, Philadelphia. 



aENEEAL. 



Great activity lias been manifested in j)liysics during the years 

 1879-'80. ISTew and important discoveries have been made, improved 

 modes of research have been devised, greater accuracy has been at- 

 tained in measures of precision, and more uniform and precise stand- 

 ards have been adopted as units. Sainte-Claire Deville and Mascart 

 communicated to the International Geodetic Association an elaborate 

 paper describing the construction and giving the properties of the new 

 platinum-iridium standard of length. The alloy was made by Johnson, 

 Matthey & Co., by fusing together 450 ounces of platinum and 55 

 ounces of iridium. After suitable working, it was drawn through a 

 draw-plate iutp a bar accurately rectangular in cross-section. Its density 

 now was 21.51 ; and it was found to contain platinum 89.41, iridium 

 10.19, rhodium 0.17, ruthenium 0.10, iron 0.06. This alloy is harder and 

 more resistant than platinum, is much more rigid, and has a high elas- 

 ticity, its modulus being 21,912,000,000, and its elongation-coefficient 

 0.0003. Its coefficient of expansion was in course of determination by 

 Mascart, the elaborate preliminary measurements only being described. — 

 {Ann. Ghim. Phys., V, xvi, 506, April, 1879.) C. S. Peirce has reported 

 progress in the comparison of a wave-length with a meter. Using a 

 Eutherford ruled plate of 340^ lines to the millimeter, three complete 

 measures of Van der WilUgen's spectrum line No. 16 gave for the angle 

 of double deviation as a mean 89^" 54' 19".5 An error of 0".4 would 

 make an error of a micron in the length of the meter. Subsequently 

 another line near the former one was selected, and a finer plate was 

 used. As a mean of six closely accordant measiu'es, the double devia- 

 tion was 90° 03' 51".45. One of these plates had been compared with 

 all the centimeters of a decimeter scale. The other is to be compared 

 with the even two centimeters of the same scale. This decimeter scale 

 was in progress of comparison by Mr. Chapman with the decimeters of 

 a meter scale. When done, a meter will have been compared with a 

 wave-length.— (Aw. J. ^cL, III, xviii, 51, July, 1879.) W. A. Rogers 

 has communicated valuable papers on metrological subjects, among 



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