HOW LIGHT EXERTS PRESSURE 33 



valent mechanically to 0*0417 x 4*2 x io 7 ergs = 

 175 x io 6 ergs. But this energy is spread over a 

 column i sq. cm. in area and 3 x io 10 cm. long, 

 the length that light travels in one second, so that 

 the energy in I cubic centimetre is 175 x IO 6 -f- 

 (3 x io 10 ) = 5'8/io 5 ergs. The pressure exerted 

 by the sunlight on the absorbing surface is there- 

 fore about 6/io 5 dynes, or, say, six one-hundred- 

 thousandths of a milligramme weight. 



