Decembek 11, 1908] 



SCIENCM 



847 



In December, 1906, in an address before the 

 American Association for the Advancement of 

 ■Science (Science, March 22, 1907, p. 451), I pre- 

 sented a table showing the daily rates of the 

 standard clock of the U. S. Naval Observatory, 

 Riefler Sidereal Clock No. 70, from February 8 

 to May 12, 1904. 



This clock was enclosed in an air-tight glass 

 ■case and was mounted in a vault where the tem- 

 perature was artificially controlled. However, 

 ■during the period mentioned it was found impos- 

 sible to prevent air from leaking into the case 

 so that a nearly uniform pressure was maintained 

 by pumping out 1 or 2 mm. a day. The mean dif- 

 ference between the computed and observed daily 

 rates for that period was O'.OIS. 



DAILY BATE OP EIEFLEK SIDEREAL CLOCK NO. 70 



clock rates of this same clock from February 6 

 to June 19, 1905. 



The following table gives the dates of observa- 

 tions, the mean pressure in the clock case, the 

 observed daily rate, the computed rate and the 

 diderence between the last two. The change in 

 the pressure is due to a progressive change in the 

 temperature of the clock vault. The computed 

 rates are obtained from the formula 



+ 0^00255 — 0=.01247 (B — 677""".5) 



— 0'.000366 (r — April 14.5) 



deduced from a least-square solution of the ob- 

 served daily rates. The mean difference between 

 the computed and observed daily rates is 0^.009. 

 The behavior of the clock can also be shown 

 from the clock corrections during the same period. 

 Each clock correction depending on ten or more 

 time stars has been reduced to April 15.0 by the 

 rate formula given above with the following result : 



COREECTION TO EIEFLEB SIDEEE.AL CLOCK NO. 70, 

 APBIL 15.0 



Immediately following the period named, we 

 succeeded in making the glass case air-tight and 

 during the past four years have had practically 

 no trouble from leakage of air. I therefore desire 

 to present to this society a discussion of the daily 



