ILLUSTRATING CARRIAGE OF MOMENTUM 63 



only sent in intermittently. The system was made 

 to vibrate torsionally in the horizontal plane. 

 In each vibration, while B was moving from the 

 source the beam was sent in, and while B was 

 moving towards the source the beam was shut off. 

 The swings then gradually increased always. Then 

 the beam was sent in while B was moving towards 

 the source and was shut off while B was moving 

 away from it. The swings then gradually 

 decreased always. The observed torque was of 

 the order expected from the energy in the beam, 

 but the effect was so small that accurate measure- 

 ments could hardly be made. The experiment is 

 to be regarded as qualitative rather than quantita- 

 tive. 1 



(3) Professor Lebedew's Experiment on Absorb- 

 ing Gases. 



We may understand the principle of the experi- 

 ment by imagining that a beam of light with 

 energy E per cubic centimetre, passes through a 

 chamber ABCD (fig. 29) containing an absorbing 

 gas, and having transparent windows AB, CD. 

 We suppose that the beam just fills the chamber. 

 If the gas absorbs a fraction a of the energy of the 

 beam it absorbs the same fraction a of the momen- 

 tum carried by the beam. Since the momentum 

 brought up per second per sq. cm. is E, that 



1 Note 4, p. 87. 



