ILLUSTRATING CARRIAGE OF MOMENTUM 57 



observed by a telescope viewing the reflection of 

 a scale in a mirror on the suspended system. 



The same beam was then directed on to the 

 same area B along CB at 45 on the other side of 

 the normal BN. The heat absorbed was the same 

 and therefore convection and radiometer action 

 might be supposed to give the same turn D and in 

 the same direction. But the horizontal light force 

 L was reversed, so that the angle now observed 

 was D L. The difference between the two ob- 

 served angles was 2L, double that due to the light 

 force in a single beam. The effect of the disturb- 

 ing forces was thus eliminated, and the elimination 

 was verified by directing the beam on to different 

 points near the circumference, when nearly the same 

 value of 2L was obtained. 



At first the experiment was tried with air at low 

 pressure in the case. Then hydrogen was used 

 instead, and the results obtained were much more 

 consistent. The following table gives the observed 

 torque in a series of experiments at various pres- 

 sures, and the torque calculated from the energy 



in the beam 



IN HYDROGEN. 



Pressure in Observed torque Calculated torque 

 cm. of mercury. in icr 6 cm. dynes. in icr 6 cm. dynes. 



r8 6-0 5.3 



i -4 6' 5 6' 2 



1 6 6-3 5-4 



1-25 6-2 5'0 



