140 



Dr. Watson on the Determination of the 



other for the observation of the mirror. An arrangement 

 was also fitted by means of which the fibre could be removed 

 and the top of the torsion-head turned so as to start the oscil- 

 lations. A side tube was connected to a manometer and a 

 Fleuss pump, which was driven by an electric motor and 

 was capable of maintaining a pressure of about 5 centimetres 

 of mercury. No attempt was made to stop whatever small 

 leaks there were, which prevented a lower pressure being- 

 attained, as the range of pressure obtained was amply sufficient 

 for the purpose. The thermometer, which projects into the 

 box of the instrument, was read through the large window. 

 A special experiment showed that the change of pressure from 

 76 cm. to 5 cm. did not affect the reading of this thermometer 

 by 0°-05 C. 



The procedure adopted was to observe the period at atmo- 

 spheric pressure, then exhaust and. observe the period, and 

 finally, the air having been readmitted, to again observe at 

 atmospheric pressure. In every case at least an hour was 

 allowed after the exhaustion or admission of the air for the 

 temperature to become uniform. 



The quartz fibre used in the comparison of the inertia-bars 

 having been broken when fitting up the vacuum- chamber a 

 new fibre had to be fitted. The following table gives the 

 observed periods. 



Cradle and Bar No. 1. 



Period at 15°. 



Pressure. 



Period at 15°. 



Pressure. 



2-5651 



75-3 cm. 



2-5646 



40 cm. 



2-5650 



75-3 



2-5645 



4-0 



2-5648 



76-8 



2-5644 



40 



2-5647 



76-8 



2-5641 



4-6 



2-5647 



72-0 



2-5641 



4-5 



2-5647 



72-0 



2-5641 



43 







2-5641 



43 







2-5641 



4-2 







2-5641 



4-2 



Means ... 2'5648 



74-7 



2-5642 



4, | 



Cradle only. 









•87049 



76-0 



•87001 



4-7 



•87046 



75-5 



•87003 



4-7 



Means ... -87047 



75-7 



•87002 



4-7 



If ti and t 2 are the periods at the lower and higher pressures 

 respectively, and K is the moment of inertia of the suspended 

 system, w r hile k x and k 2 are the moments of inertia of the air 



