315 



L=i4. M— 391^84 + 4 X 8-^7 = 4 .1/ — 357/M6. 

 The value of L had been detertuiiied several times before at the 

 "Hureaii iiiteriiatioiial des poids et mesiires" at Hi-eteuil ; on these 

 occasions the following valnes were obtained, leaving out the some- 

 what uncertain correction for the "change in the molecular equilibrium". 



1903 March 4 i)/— 377^6 Breteuil 



1904 June —373 .5 

 1907 February —363 .7 

 1909 February —356 .8 



1909 December —358 .4 



1910 December -357 .2 



1911 June —355 .3 

 1911 Sept.-October - 358 .4 

 1913 April —348 .7 

 1913-14 Dec-January —357 .2 Delft. 



It appears from these numbers that during the tirst years up to 

 1909 the bar increased in length. From that year onwards the length 

 seems to have remained practically unchanged; only in April 1913 

 a further very marked increase in length shows itself, of which, 

 however, no trace is found in the measiu-ement made by us. In 

 view of this contradiction a new determination at Breteuil of the 

 length of the measuring-bar is desirable; col. Lallemand, chief of 

 the geodetic department of the Service géographique de l'armée, and 

 Monsieur Benoit, Director of the Bureau international des poids et 

 mesures at Breteuil, have both promised to undertake this comparison 

 shortly. 



Postscript. A few weeks after the meeting of our Academy 

 I received from Monsieur Benoit a letter in which he communicated 

 the results of on elaborate investigation concerning the length of the bar 

 of the French base appai-atus. He and Monsieur Maudet comi)ared 

 in the Bureau at Breteuil first that bar and three other ones each 

 with the prototype and afterwards the four bars with one another. 

 Benoit found as final result for the length of the Freiu'h bar, with- 

 out correction for the change in the molecular equilibrium: 



L„ = 4 J/— 348/^23, 

 almost exactly the same value as that found in April 1913. 



On the average the length determined at Breteuil is therefore 8". 7 

 greater than that determined at Delft. 



In order to find what may be the causes of that ditference I 



