proceedings: philosophical society 475 



The Secretary read the call for the first meeting of the Society arid 

 the list of charter members. 



Mr. William H. Dall delivered an address on The origin and early 

 days of the Philosophical Society of Washington. The address will prob- 

 ably be published in full in this Journal at a later date. 



The Secretary read a letter from Mr. G. K. Gilbert, the second 

 oldest member of the Society, conveying his best wishes and expressing 

 regret that it was impossible for him to be present at the meeting. 



Mr. F. W. Clarke delivered an address on The development of scien- 

 tific societies in Washington. 



Interesting reminiscences of the early days of the society were offered 

 by Messrs. Howard, Farquhar, Wead, and Bauer. 



The 790th meeting was held at the Cosmos Club, May 12, 1917 

 President Buckingham in the chair; 28 persons present. The minute^ 

 of the 789th meeting were read in abstract and approved. 



Messrs. I. G. Priest and C. G. Peters gave an illustrated paper on 

 An interferential method for measuring the expansion of very small sam- 

 ples. This paper describes a method requiring as a sample only one 

 small pin about 5 mm. in diameter by 10 mm. long, or even of smaller 

 dimensions. Briefly the outline of the method is as follow: 



1. One of a pair of nearly (but not quite) parallel interferometer 

 mirrors is supported at one point by the sample and at two other 

 points by a standard substance of known expansivity. 



2. Any difference in the expansivity of the sample and the standard 

 causes the interference fringes to change their widths with changing 

 temperature. This change in width is measured, and from it the rela- 

 tive expansion is computed. 



Noteworthy advantages of this method over the Fizeau-Pulfrich 

 method are: 



1. Smallness of sample required. 



2. Elimination of the difficulty of obtaining the interference fringe 

 data. It is not necessary to count the passage of fringes during the 

 temperature change nor to make measurements on fringes of more than 

 one wave length. 



3. The very troublesome correction for change in the refractive index 

 of air with temperature is eliminated. 



It is expected that a fuller description of this method will appear 

 later as a Scientific Paper of the Bureau of Standards. 



Discussion. The paper was discussed by Messrs. Buckingham and 

 Humphreys. 



By invitation Mr. J. D. Edwards gave an illustrated paper on The 

 rapid determination of gas density. 



Because of their importance in connection with orifice meter meas- 

 urements and other technical and scientific problems, methods for the 

 rapid determination of gas density have been investigated at the 

 Bureau of Standards. 



