ILLUSTRATING CARRIAGE OF MOMENTUM 6? 



been a difference of pressure p (that to be deter- 

 mined) on the two ends of the piston, and it would 

 have been pushed along until the twist of the fibre 

 introduced a torque equal and opposite to that due 

 to/. But of course this is an impossible ideal, and 

 to avoid sticking friction the piston had free play 

 in the cavity and the gas streamed past it to a 

 small extent and circulated round the two 

 chambers. The difference of pressure on the 

 two faces was therefore somewhat less than /. 

 The pressure lost was determined by a subsidiary 

 experiment into which we shall not enter. 



The value of I scale division deflection was 

 found in the usual way from the time of vibration 

 of the torsion rod loaded and unloaded. 



The gases chosen were Carbon dioxide (CO 2 ), 

 Methane (CH 4 ), Ethylene (C 2 H 4 ), Acetylene (C 2 H 2 ), 

 Propane (C 3 H 8 ), and Normal-Butane (C 4 H 10 ), all 

 highly absorbing. With the absorption of light 

 the gas in the chamber became hotter, and slightly 

 hotter, no doubt, in front than at the back. There 

 was also local heating owing to the need of con- 

 centrating the beam on the gas instead of making 

 it parallel. Thus there was a tendency to the form- 

 ation of convection currents. To reduce these 

 each gas was diluted, usually with an equal volume 

 of hydrogen. Hydrogen is practically transparent, 

 but is comparatively a good conductor, and it 

 served to equalize the temperature in different 



E 2 



